The chassis cover is held in place by 18 machine screws that need to be removed and replaced in order to fit the tubes. Also under the cover are the speed controls for the twin cooling fans mounted on the rear panel. Putting safety first and assuming a worst-case scenario, ARC sensibly set the fans to the highest of their three speed settings to provide maximum cooling -- essential if the amp is to be placed in an enclosed space or rack. But if you are going to use the amp out in the open, you might find the fans unacceptably loud. I used the 150 SE in two different free-standing situations: as a pair of the stereo amps (greedy, I know) driving the Wilson Sasha 2/WATCH Dog system in my larger main listening room, and in a simpler, two-channel setup in my smaller room, driving either Vienna Acoustics' Liszt floorstanders or MartinLogan’s (shockingly good) Motion 35XT stand-mounts. In both cases I found the fan noise intrusive. Just as well that you can reduce the speed then -- except that to do so means removing and replacing the cover all over again. The moral of this story? Consider fan speed at the same time as installing the tubes. If you are going to be using the amp in a free-standing setup or close to the listening position, then tack the cover back in place with four screws and fire it up to check the noise level before battening down the hatches.

As with almost all large amps, the 150 SE's chassis cover is also largely unsupported, meaning that it can vibrate. I found that adding a long HRS damping plate to the spine of the 150 SE’s case delivered a worthwhile improvement in terms of the resolution, microdynamics, separation, dimensionality and the blackness of the background. The soundstage went deeper, was much more clearly defined, and the sound was more intimate, more immediate and more communicative. If you think that sounds like a significant benefit, you’d be right. Once again, the good news is that the fix is easy -- and easily investigated. It’s something the company is aware of but they are reluctant to simply apply damping pads to the underside of the cover, given the effect of elevated temperature on the performance and longevity of such pads. A constrained-layer panel (similar in effect to the HRS plate) is a possible solution, but that will take time and, inevitably, money. In the meantime, if I were using or considering this or any of the other ARC amps, I’d want to hear what the HRS damping plates can do for their performance.

One final wrinkle involves the output taps. Like most tube amps the 150 SE offers a choice of 4-, 8- and 16-ohm outputs, allowing the user to match the amp to the speaker load. Given that speaker impedance characteristics are rarely constant and frequently complex, this isn’t a straightforward decision. Indeed, in many cases shifting between an amplifier’s 4- and 8-ohm taps will be a swings-and-roundabouts exercise, with each offering its own strengths and flavor. Not so the Reference 150 SE. Here I found that the choice of impedance tap was not only critical to maximizing performance, the best choice wasn’t necessarily the obvious one. So, both the Liszt and the Motion 35XT sounded significantly better connected to the 4-ohm taps -- exactly as their specs suggest. The Sasha 2s, on the other hand, definitely prefer the 8-ohm taps, despite a 4-ohm rating and a minimum impedance value that hovers just above 2 ohms. It’s another aspect of setup that’s easy to overlook but almost as easy to check. With the Reference 150 SE, it can make a real difference, so it's worth taking the time to make sure you have the best setting.

The fact that I’ve used so much space on the housekeeping involved in getting the best out of this amp should tell you two things. First, it is capable of remarkable resolution, and second, it is also capable of remarkable musical performance. The same qualities that reveal just how big a difference something like the HRS plate can make dig deeply into the signal, revealing the very nature of the musical performance encoded within. Was my first impression of the 150 SE as startling as the first time I heard the D115? No -- because my expectations have shifted accordingly. Does the current amp exhibit the same musical and sonic DNA as the D115? Yes and no. Yes, in that so much of what makes this amp such an engagingly musical performer is linked to its sense of presence and dynamic authority; no, in that the 150 SE brings a whole host of musical and sonic capabilities to the party that the D115 couldn’t even dream of.

This isn’t just evolution -- this is transformation. The 150 SE matches the dynamic power and impact that made the D115 such a standout and shares that amp’s incredible ability to swell and expand with the music when necessary. But while the 150 SE’s core capabilities remain firmly rooted in its ability to deliver convincing dynamic range and weight, coupled to a rich and natural tonal palette, it now augments those qualities with a level of spatial, textural, harmonic and dimensional resolution that adds shape and perspective to the picture. Vocal and instrumental accents are much more apparent, each performer’s character more identifiable, the overall shape of the song clearer and musically more effective. If you want a quick reality check, just try a round or two of that old audiophile favorite "name that artist," and see how easy this amp makes the task of identifying the singer performing any given cover number.

That sense of immediacy and musical intimacy works wonders with small-scale acoustic rock/pop or jazz recordings. The Reference 150 SE effortlessly captures the breathy texture and individual tonalities of the horns, the shape and pitch of bass notes, the pace and placement of drum patterns and piano lines, integrating them into a musically cohesive whole. At the same time, large-scale orchestral dynamics are delivered with a sense of substance that makes them truly dramatic -- and without allowing the brass and percussion to climb forward and trample all over the strings. Instead the orchestral image is held stable while the music swells out from it, expanding into the acoustic space -- the music just gets louder, not bigger, and it doesn't come not further forward. Setting up a decent soundstage with good dimensionality and a natural perspective is one thing; locking that soundstage in space, maintaining its dimensions and holding the instruments stable within it while also responding to the dynamic demands of the musical program, that’s quite another. It doesn’t matter whether you are talking about a jazz quartet, a girl and a guitar or the Berlin Philharmonic, as soon as the stability of the soundstage collapses, or even wavers, the fragile illusion of life and presence, the sense of performance and performers in your room, goes with it. Few amps are truly capable in this regard -- and most of the ones that are boast massive power outputs and a sound that crushes the vitality out of even the most spirited performance. The Reference 150 SE can’t match the carved-from-solid definition of a really big amp, but it does hold the acoustic space rock steady, the instrumental images stable in a way that is both surprisingly rare and musically satisfying.

It’s this ability to resolve without pulling apart, to separate without dismantling, to hold stable without holding too tight that characterizes not just the Reference 150 SE but ARC’s Reference products as a whole. Does that make them perfect -- and this the perfect amp? No, because they are not without character. There are certainly amplifiers that achieve even greater levels of transparency and control than the Reference 150 SE, especially at low frequencies, and let's not forget the existence of ARC’s own Reference 250 SE and Reference 750 monoblocks. There are amps with wider dynamic range, better microdynamic definition, more emphatic temporal cues and a more purposeful sense of musical direction. But in almost all cases, those qualities come at the expense of the overall sense of unforced musical coherence, the convincing sense of a musical whole that flows so easily from the Reference 150 SE -- especially when hung on the end of the Reference 10 line stage. It leans on the shape of phrases rather than the shape of individual notes, but then it’s called a sentence for a reason, and that’s exactly what the Reference 150 SE makes -- musical sentences that make musical sense.

I could dismantle the 150 SE’s performance and slice it up into audiophile sound bites. I should point out that where it deviates most from neutral is at the bottom end, where it values the weight and center of the note over leading-edge definition, a design choice that is intimately connected to its relaxed and communicative qualities, but one that also places an extra emphasis on precise speaker setup -- and confuses the issue when it comes to direct comparisons. But instead I’ll just observe that the way to listen to the 150 SE is to put it in a system, pay attention to the housekeeping details outlined above, adjust the position of the speakers to match its balance, and then settle back for a good long time. That’s when you’ll really appreciate just how easy this amp is to listen to -- and how easy it makes listening. That uncanny combination of musical insight, substance and integrity is what makes the Reference 150 SE another genuine benchmark product. Don’t misunderstand that terminology. "Benchmark" doesn’t mean best. What it means is a product with the overall balance of musical capabilities that the competition needs to match or beat.

And the most important word in that sentence is balance. A single-chassis amplifier with enough power to drive all but the most demanding speakers and do so with the musical authority to really deliver on the promise of high-end audio, the Reference 150 SE is capable of delivering so much music in such a coherent way that you simply don’t notice what it doesn’t do.

Any complaints? I’d really like meters. The Reference 75 SE has meters; so do the Reference 250 SE monos. But as ARC point out, the Reference 110 didn’t have them, nor did the Reference 150 (partly, I suspect, because all those reservoir caps are taking up the space immediately behind the front panel) so as an upgrade on the 150, the 150 SE misses out too. And just to put the tin lid on it, the D115 had no meters either, so perhaps it was never going to be, especially as the Reference 150 SE embodies so much of what left that older amp lingering long in my musical memory -- and then some. This amp doesn’t just fill its predecessors’ shoes; it needs a bigger size.