Setting aside the debate over audiophile tube amps versus solid state amps, I think it is fair to say that while the amp matters A LOT in the quality of the sound, modern electronics being what they are, it doesn't take much to build a really really good amp these days. There's no design "magic" left to be paid for. Unlike speakers/monitors, with amplifiers, a handful of basic designs are simply tailored to whatever power output is desired and then choices are made regarding the quality of the case, connectors, knobs, pots, etc. The components that actually affect the sound are really simple and dirt cheap (Op amps transistors, resistors, and capacitors). Here again, I'm ignoring a few audiophile black magic debates, particularly in saying capacitors are cheap. The perfect examples of what I'm talking about are the operational amplifiers. We're still using the same chips that were developed in the late 70's and early 80's. At the time they were nothing short of revolutionary in providing gain and signal to noise ratios that previously could only be imagined, but once the National Instruments, Analog Devices, Fairchild Semiconductor and Texas Instruments guys churned through a decade of iterations, the results just weren't going to get any better. So for way less than a dollar you can buy a really, really, really.... really, really... really good audio op amp.
The one area where there is still some technology competition happening (sort of) is in power supply design. The result is high power amps that don't weigh 300lbs. Not much magic left here either, but it does take some art to put a switching power supply (lightweight) into an audio device and maintain state of the art noise levels. For all I know this competition may already have ended as well, but if you see higher prices based on "technology" for lighter weight amps, there may be some legitimate basis for that.
Net-net, you need a good amp to make good sound, and good is never free, but beware of marketing claims and crazy prices based on the sound quality. Look at the specs to make sure you're looking at a decent amp and then look to ruggedness of the case and the quality of the connectors, knobs, pots, etc., and maybe weight, because the audio circuit inside is going to be a near carbon copy of every other amp your looking at.
J
PS - Before anybody cites other things I've written about tube amps. The debate I'm talking about here is tube amps for audio, not for guitar. For guitar amps there is no debate. Tubes are simply necessary. That's because as pure "amplifiers" they have a certain brand of imperfection (aka distortion) that is an essential part of the sound we associate with guitar. Solid state guitar amplifiers typically are designed to mimic this pleasing distortion with less than good results (IMHO). Many audiophiles make the same claim of well designed tube amps for audio. I've never had access to any of those amps for comparison. I'll admit I'm a bit of a skoffer, but I'm not saying there isn't anything to that. I am saying there's no magic to be found between one "good" solid state amp and another, though, in terms of sound quality.
