Studio Power Amps

I have an old Alesis RA150...no fans, totally inexpensive, and works great.  I think there are actually a fair amount of amps on the market without fans, if you start looking.

If someone can hear the difference between decent amps, not pushed to their limits.....well...I'm not sure that's possible.  If you're nostalgic...get an old Crown D150a or similar.   For a low wattage amp, I don't see the need to spend $300 or so, unless you totally shy away from Asian made.....ya know...that big country with their name starting with a "C"

If you follow the link I posted above it explains the difference in classes of amps. It's complicated and do not even attempt to understand it fully without a degree in engineering.  
But this I gleaned from the text: 
Class A amp can have the best overall sound but they are inefficient so they generate a lot of heat and therefore the loud fans. Class D is a newer technology and are very efficient so they  not only weigh less, they don't generate much heat therefore will be quieter and often passive.
A passive Class A amp will probably be 80 to 100 watts. My Yamaha P2100 is rated at 120 Watts and is passive but it has a huge heat sink around it and I keep my feet warm with it under my desk. 
 
Class D is what is found in modern powered PA speakers and monitors. The reason being the weight and heat of a Class A or B would not be feasible. The speaker would catch on fire. 
Sound quality can be an issue with any class of amp if poorly designed. There are certainly plenty of badly designed Class D amps. ( dare I say the B word) 
Class D works best with a feedback loop to control it and matching the Amp to the speaker inside a cabinet, bi amping and all that is what make powered speakers a very good choice.  I have ditched my passive PA system. 
 
And it is very tempting to go powered with a new set of Yamaha HS80's. They say they are a close replacement.  It might cost me the same to buy a good power amp.  
But I hate to give up my NMS10's so therefore the research. 

I have a variety of older amplifiers from Crown, QSC, Haffler, and Heathkit (yeah, you read that right!)

I've used a lot of others over the years. For the most part imperfections & non-linearities in the monitors swamp anything going on in the amplifiers. I have tried a couple of inexpensive Class-D amplifiers (in fairness this was several years ago) and I did not like them. I could hear the difference between them and the dinosaurs I own, and not in a good way.

The only two amplifiers I think I can identify - in a positive way - with a blindfold are the Heathkit and any Bryston. They sound nothing alike, the Heathkit sounds pretty, and makes most monitors sound pretty, probably not what one wants in a studio setting! The Bryston makes most monitors sound better. I can't explain it, and I'm not in a position to spend that kind of money on an amplifier, so I don't even try<G>! (Actually, NAD used to make some really good sounding amplifiers, but they had a nasty habit of overheating in studio settings, and as someone pointed out, fans are not a good idea in a studio!)

As a rule I try to avoid Class D amplifiers. That's become nearly impossible, and if you are using monitors with built in amplification I think it might actually be impossible.

But for the most part any competently designed amplifier should sound good - there aren't a lot of great new ideas in amplifier topology for Class A or Class AB amplifiers. You might be able to detect a difference between an amplifier that uses feedback vs one that doesn't, but I'm not sure which one I'd pick as "better".
 
The only thing you have to remember is that no one ever blew up a loudspeaker with an amplifier that was too big<G>! Other than that I'd just get a good Class AB, MOSFET amplifier, and there are lots of those around.

wst3
I have a variety of older amplifiers from Crown, QSC, Haffler, and Heathkit (yeah, you read that right!)

 
I'm sure there are many people on this forum that built Heathkits. I know I did. My first home stereo amp was a HH Scott kit with "Fire Bottles". Had a great sound! When I upgraded my stereo years later, I made a tough screen protective cage for it and used it as a back up amp for my band. Our base player's amp failed one night, (it was just a filter cap) and the Scott saved the evening.
 
Walt
 

my Allied (Ratshack) still plays NPR in my shop.

batsbrew
i like, and use, hafler amps.

 
And, if you are the curious type there are some great modifications out there... never going to sound like a Bryston, but you can make them sound even better than they do stock.
 

batsbrew
don't get more than you need..
but get a little more than you need,
if you know what i mean

Yup - always get just a wee bit more than you need. My basic, never violate it, rule of thumb is 20% more power, which works out to a paltry 1.5 dB. In reality I like to shoot for about 3 dB headroom above the loudspeaker rating.

 
Personally, since I couldn't afford both high quality monitors and high quality hi-fi amp/speakers, I'd go with the latter since I listen much more than I mix.  I've never been convinced of the argument that hi-fi sounds too good to be useful for mixing, where you want sterile, neutral, harsh -  but predictable.  I'd much rather learn to translate the sound of great hi-fi speakers to a more general purpose sound - while enjoying everyday great sound reproduction - than translate/suffer recommended monitors that can't be used for anythings else.  You can be sure that the amp in powered monitors is about as cheap as it gets.