audio1326 - I’ll chew a slice of your query. I investigated your question for affordable amps for the CS2.2 and landed on Adcom, and specifically the GFA555. In that process I sorted out some considerations, summarized here.
The GFA555 was designed by (the legandary one and only) Nelson Pass, as were the 535 and 545. Simple, low parts count, high current circuits which depend on high-quality parts. First generation boards were built in Japan with ’best’ available parts. Later runs moved to Taiwan with lower quality parts. I don’t know the serial numbers. There is even better Japanese transistor sets and components available now. (See Audio Upgrades below.)
The geriatric capacitor problem is a real but manageable problem. A competent electronics shop can measure performance of the caps. If they’re good, they’re good.
I bought a GFA555 and a GFA555 mkII which is generally considered ’more refined’. My journey landed on the original (Nelson Pass) GFA555 being superior and upgradable (by reason of its intrinsic superiority). I would consider the added cost of a 555 over the 545 or 535 to be a no-brainer. Transients require power and more is better if circuit finesse is not sacrificed. The 555 gives up nothing and gets more power.
The winning aspect is that Jim Williams of Audio Upgrades in Carlsbad CA upgrades this amp to two performance levels. The upper level adds world-class metal film resistors to the basic job. Cost is in the $hundreds, not $thousands. Very highly recommended.
Furthermore, Bill Thalmann of Music Technology is reviewing my Jim Williams GFA555 upgrade to recommend and test further audiophile enhancements. I intend to recommend this amp with upgrades (wherever the particulars fall) to Thiel Renaissance clients as a high value, high performance amplification solution.
I will not comment on other contenders, having no personal experience - with the exception that Prima Luna is an outstanding performer for its price. (As long as you enjoy tube rolling.)
