I've had an Adcom 555 since about 1985. I've used it and abused it. It's run ESLs, dynamics, magnetic planars. It's been run full-range and as a subwoofer amp. I've used it in my living room and on my test bench. All it's ever done is work perfectly.
I have used a Adcom GFA 555ii on a number of speakers (horns to panels). The series ii has the advantage in that the later models (not all however) had by-pass caps on the big power supply caps.
This is an advantage (and can be retrofit in the original series amps). With new by-pass caps, what I noticed was that the bass was more solid and articulate (it is hard to choose words). However, while one friend agreed with my description, he also said that the bass now sounded less full and lush.
This is just a caution that not everyone prefers the modification. But it is cheap and reversible. Search the threads for selecting the proper value cap (I used non-polar electrolytics of 22uF on each big cap)
Thank you all for the advice! It seems that everybody prefer Adcom over Hafler. I guess Adcom is the way to go then! Is there any advantages (besides the bypass caps) that the GFA-555 II has over the GFA-555. I heard the series II has a potential problem of leaking caps damaging the circuit board.
I have had 2 555's, and the sound is very dynamic. The bass slams, and the highs are not grainy at all. I think Adcom got a bad rep for having grainy amps from the Adcom pre's hooked up to them. The only real good Adcom pre is the 750. Others have liked the 650. I had a 500, and it was pretty bad. I have yet to hear a pre with the slam the 500 had, but the rest of the SQ was pretty weak. From other Adcom owners I have chatted with they prefer the 555 over the 555II.
A Soundcraftsmen sounds better then an Adcom and was even reviewed better in Audio magazine (i think) some years ago. But the SC has a fan that kicks in when you really run it hard.
The adcom would be my second choice and the Haflers, they have never been my favorite as stock out of the box. maybe after mods there better??
The haflers are the surprising thing as there design is actually close to the SC design. the exception being the double differential input stage VS single Diff for the SC. same Toshiba mosfets, same Motorola Driver transistors. very similar circuit. But the SC's blow them away! The input stage must really make a major difference???
Some people dont like the SC regulator system. I admit the Phase Control Regulation system is a bit odd i guess. I have a SC i am rebuilding now into a pair of mono blocks that i will be using with a regular PSU and see how it sounds.