Amps which are Great, but either Unknown or Underpriced

I 2nd the Power Modules Inc.  Dave Belles is a real master and deserves the credit that guys like Nelson Pass get.  Dave is right up there with the true masters of the audio art.

Quicksilver Audio!

I second Clayton, and would add SMc Audio upgraded McCormack amplifiers and preamplifiers 

Get yer Williamson/Ultra Linear on...Jolida rarely gets anything reviewed and consequently is one of the best kept secrets (except my grabbing any opportunity to rant about these things) out there. I have a 4 or so year old "factory upgraded" 502p amp that is the quietest (seemingly zero hum or noise even if you stick your ear on the speaker) amp I've owned including a lot of SS amps, and festooned (!) with KT150s and some nice small tubes it sounds like the friggin Giant Killer it is. The fact that these things are inexpensive seems to be the only thing wrong with them…I've spent a lot of time listening to much more expensive amps from Rogers, VTL, ARC, etc. (albeit usually not in my rig with a few exceptions) and the results are generally "meh."  Primalunas get reviewed often and although closer to the price point of Jolida's  amps, and fitted with auto bias circuits and other interesting features, they generally are less powerful than a Jolida costing about a third as much, and don't sound any better to my ears (I tried a borrowed "HP" version…when I switched back to my 502 I felt the Jolida simply sounded better, although I have no "triode" switch). 

That is funny you mention Jolida. My technician works on Jolida gear and he tells me he likes it better than the mega priced products. He said a lot of people don't buy it because they think it is to Inexpensive.