You can find some info and specs and manual pages for the unit on:
I haven't heard the new series 3 amps, but that page also talks about srm-1 vs them, and SRM-1 driving not lambda frame ear-speakers.
From what I understand, that makes it good value from about 400 to $700 depending on version, condition and mods. (unlike the other reviewer suggesting $300).
I purchased my SRM-1/mk2 rev c (pro + normal sockets) from a fellow head-fier and it was fubar. Initially the unit went to the Sydney 'authorized Stax repairer' who claimed to have fixed the problem and 'tested it on both outs/everything worked' but somehow didn't notice the dead bias resistor...yea... To add insult to injury they changed only three out of four electrolytic caps....
Took me almost a month and a lot of help from Kevin and Birgir to fix it (mostly figuring out how to use a multimeter and a soldering iron without killing anything, waiting for parts and figuring out that my MD11 source was faulty and putting DC into line out). The end result was replacing the electrolytic power section caps (not good after 30 years/the aluminum bits at the top were cracked/etc) and diodes, a dead bias resistor and bias diode and some 650v film caps (4 power ones upgraded to 1KV as per schmatics) + 2pf 500v caps (upgraded to 1KV on spritzer's advice).
I've also talked to Birgir and he says the driver unit can be easily rewired for balanced ops (fit the minixlr and replace a couple of resistors), which is a plus for some. To top it off, the build quality is very solid. All the parts are discrete, and pretty user serviceable (and the schematics are floating around on head-fi), if you don't mind working with HV.
The results from a refurbished SRM1/mk2 are outstanding:
Clear audio across the whole spectrum - bass, mids, treble - from what I can tell - no roll-offs or bloat peaks - very neutral ss. Wonderful detail levels.
Enough power to easily drive my SR-404 to far beyond where my SRM-300 and SRM-252 were clipping (<=300v max out, clipping psu rails earlier). According to the faust3d page it's even suitable for Omegas in absence of enough cash for an SRM-7x7 or KGSS/Exstata .
I initially had a grounding issue with my DACs via USB->RCA, but then connected the amp via a jap 2 pin cable (probably what it's intended to be used with anyway) and now everything is good. Do note, the later models no longer have the voltage switch. you may need to get the unit rewired for your voltage (not hard, soldering 2-3 wires/plenty of help in the stax thread for that)
I'll soon have a chance to try SR-507 with it and compare it to Transistor-Amp-V3 and will update the impressions here. In the meantime, I still highly recommend the unit, but be sure to replace the electrolytic caps and get a non-fubar unit (if the transistors are dead, some of them seem to be pretty annoying to replace).
here are some pics:
Replaced parts (less large power caps not shown and a bias resistor )View attachment