I'm not too familiar with the new Adcom...but the old Adcom was my first foray into what was high-end for me. I started with the GFA-535/555 and moved up with every 200 watt series thereafter...ending with the 5500 series which I still use on my subs and a 555MKII for the bedroom. They are very capable amps that drive just about anything and can run on the warm side.
Here is a little snip from one of the original Stereophile reviews on the GFA-555 (older versions of the 5500/5503)...which was very complimentary:
Stereophile Magazine review By Anthony H. Cordesman, April 1985.
I am reluctant to call any given transistor power amplifier a "best buy" or "breakthrough." From my talks with designers and other audiophiles, it is clear that the state of the art in power amplifiers is about to change. From where I stand, the Adcom GFA-555 is the first sample of this new wave. It is so clearly superior to past amplifiers in the low- to mid-priced rangeâ€”not to mention most amplifiers two to three times its priceâ€”that I can unhesitatingly recommend it for even the most demanding high end system.
The GFA-555 does everything well, and most things exceptionally well. It provides superb, well-controlled bass with far better speaker load tolerance than most amps. Its midrange and treble are remarkably low in coloration. There is no hint of hardness, and none of the loss of inner detail common to transistor amplifiers. With the exception of the Krells, I have never heard a more detailed, natural, and extended upper four octaves in a transistor amp. The Adcom may even be a legitimate rival to the Krell; it's brighter and more dynamic; and somewhat more open.
And, like the Krell, it gives the impression, on really good material, that the amplifier simply isn't there. Nor is the Adcom romantic or sweet, like New York Audio's new Moscodes. Rather, it offers natural upper-octave detail that the latter miss. Other amplifiers have similar upper-octave performance, but I unhesitatingly recommend the Adcom over the very stiff competition from Tandberg and Threshold.
The Adcom's soundstage is sufficiently superior that even those who claim all power amplifiers sound alike might hear the difference. It comes very close to the better tube power amplifiers in providing detailed, stable, realistic imaging with natural depth. It is not an Audio Research D-250, but is extraordinarily holographicâ€”I suspect almost embarrassingly so. This kind of soundstage has previously cost at least $2000.
I am also highly impressed with this amplifier's dynamics. Once again, it is not going to survive a one-on-one with the Audio Research D-250 or Conrad-Johnson Premier Fives, but it rivals any transistor power amplifier in its power class that I have heardâ€”including high-powered receivers or amps with trick power suppliesâ€”at any price. It provides these dynamics into virtually any load without bloat, restriction of sound, or change in timbre. For all the nonsense published by most manufacturers about driving complex loads, this amplifier actually delivers..."