Pros: ONE OF THE BEST STAX TRANSISTOR AMPS, ADJUSTABLE VOLTAGE
Cons: JUST THAT THESE ARE QUITE OLD
After some years of seeing other headfiers rave about these amps, I finally got a B series model and found that indeed it had advantages over my later model SRM3 amp. I was mostly concerned with running a set of Sigma pros and Sigma/404's which are the least efficient Stax phones and they definitely sounded better with the SRM1Mk2 than my more modern SRM3.
For some reason Stax reduced the voltage swing on the later amps compared to the SRM1Mk2, reducing it from 370 to 300 volts. Stax kept this lower voltage swing on most later transistor and tube amps although the most recent designs seem to be pushing it up again. The 717 for example runs 450 volts and I believe some other new amps are back at about 340.. This may be why I hear better dynamics and air with the SRM1Mk2 than the SRM3.
I liked the SRM1Mk2 so much that I sold one of my 2 SRM3's and bought a second SRM1Mk2. The second Mk2 was an A series model, meaning that it was even older. The C series being the most recent models. I could hear no difference between the A and B models.
The Stax site shows that the original SRM1 dates back to 1979. http://www.stax.co.jp/Export/History.html
It has both high and low bias sockets. The SRM1Mk2 was first released in 1982. I do not know what differentiates it from the non-Mk2. The very last SRM1Mk2's dropped the low bias socket. and have 2 high bias sockets. Some models are champagne colored, some are black.
To me the low bias socket is a plus, since I have some low bias phones. If you really need 2 high bias outlets you can use a Stax 3-way adapter (but this needs an extension cord) or make can your own adapter.
Another advantage over later Stax amps is that it is easy to change the input voltage so you can take say a Japanese or British model and switch it over to 117 volts with a simple adjustment of an external plug. Stax stopped offering this plug in later models, probably to close down the grey market of imported models coming in outside of the Stax dealers. Subsequently they have cut wires around the power transformer so that it is even more difficult to make internal modifications to the input voltage of their most recent amps..
I have heard some users contend that the Mk2 tends to harshness. If so it's not obvious on the Sigma Pro, Lambda Signature and Sigma/404 I normally use with these amps. I rate this amp at 4 stars in audio quality where the $5,000.00 BHSE super amp would be 5 stars and a used 717 would be 4 1/2 stars.
The only real drawback with these amps is their age. Some suggest replacing the large capacitors on such old amps. I haven't done that myself so I can't say if it would make them sound better.
On balance, I think it is one of the better Stax amps and I would recommend it over the more recent amps that give a swing of only 300 volts. If you can get a decent one for $300.00 it is a good amp to have.