What qualities do you look for in a good amp?

Look for an amp that doubles it power (watts) as the ohms are halved. This is a sign of a stable amp that will drive almost any speaker load.

IE:
200 watts @ 8 ohms
400 watss @ 4 ohms
800 watss @ 2 ohms etc.,etc.

Like Rockvirgo said, the heavier the amp, usually the better... this translates into a larger power supply and more $$$.

Short list to start with: Audio Reserach, Bryston, Krell, Mark Lenvinson, Pass Labs.(only to name a few) These are all high quality, well made amps, by 1st rate manufactures.

The final choice is what sounds best to YOU, not the BS the salesperson tells you. Good Luck!

I agree with rockvideo.Let me add some specifics.Some of the amps mentioned by Ig40 did not work for me.You need an amp with big dynamic range no feed back and reputed to be reliable.I have been happiest with Gryphon and YBA amps.

All excellent responses. There are many factors that go into purchasing an amp. You have to ask yourself a few fundamental questions.

1. How much power do I want?
2. How much heat do I want to put up with?
3. How much space do I have for the amp?
4. How much do I want to spend?
5. Tube Vs Solid state Which sounds better TO YOU!!
6. Do I prefer a neutral sound or a euphonic warm sound?
7. What are the other components in the system and how can
system synergy be achieved?

If you aswer these questions honestly you can narrow down you search to a few manufacturers and then auditon them!!

What to look for. No particular order.

1) Customer service dept readily available.
2) Stable reliable company.
3) Few models used up for sale.
4) Input impedance high enough to allow use of tube pre-amp.
5) Power rating stable into 4 ohms.
6) Protection circuitry that prevents failure due to clipping.
7) Ability to throw up a open sounstage and image precisely.
8) No excess energy in the bass/upper bass/lower midrange.
9) Highs that sound real without ripping your ears off.
10) Tonal accuracy in the midrange that produces palpable voices.
11) Binding posts that allow use of spades/bananas/bare wire/bi-wire.
12) Adjustable input sensitivity.
13) Reliable proven design.
14) Balanced circuitry that is truly differential.
15) Good resale value
16) Available in black or silver. No glitzy appearance.
17) Moderate heat to keep your room cool in summer.
18) High dynamic headroom.
19) Quick warm up time- under 1 1/2 hours to enjoy the sound.

Anything else??

I too was on the "Truly Balanced" path, courtesy of my Rowland Model 2. I even worshipped at the portal of "BPS" (battery power supply). This amp was "IT" for me.
Then I heard an Aleph 3. Class A, single ended, sounds funky on warm up. Heats the room. But on my Apogee Mini Grands, my Montana SPIIs, and my custom 2 way monitors, the Aleph amps sound better than anything I've tried.
Class A means NO headroom, that's OK, I have never clipped my Aleph 2s on my Apogees. 160 watts Class A does not sound at all like a 200 watt class A/B amp. It just sounds better.
I also own an Adcom 5500 stereo amp, this is THE most underatted amp available today, in my opinion. It runs quite hot for a class A/B amp. Nelson Pass had a hand in it's design, & it shows. You can try one of these used for around $600.00.