You can find many glowing reviews of S300 or M300. I use Rowland 102 that is based on the same module and it sounds fantastic. Module used is not ASP250 but 200ASC (just for the reference). Highs are wonderful extended and not "splashy". Mids are out of this world with extraordinary mid-range speaker in my Hyperions HPS-938. Sibilants are clear and undistorted. One thing you need to check is minimum impedance of your speakers - should be not lower than 3ohms (Ref1000 can drive 2 ohms). You might consider S300 or Rowland 102 since module inside has its own power supply thus the only difference between S300 an M300 is single case instead of two cases. Rowland has better heavier chassis (milled from the block of aluminum) and additional instrumentation amp (based on THAT1200) that increases common mode rejection while also increases input impedance from 10k to 40k. Jeff Rowland is very proud of this amp. Sound is more toward tube than transistor IMHO but I found it slightly lean in the lower midrange - upgrading speaker cable from AQ Indigo to Acoustic Zen Satori fixed it completely. My system: MacMini, Airport Express, Benchmark DAC1, Rowland 102, Hyperion HPS938. It sounds very neutral in spite of Benchmark being purposely designed not to be warm sounding.
There was comparative review of M300 and REF1000 in Stereo Times:
http://www.stereotimes.com/amp111307.shtml
This part of the review shows the difference between two:
"It is to be expected that these two amps will sound very much the same, as indeed they do, slightly better bass being a generally distinguished trait of the REF1000. Subtler differences are elusive. So when I suggest that the M300 is a sweeter and airier amplifier, and that the REF1000 does a better job of pinning down the positions of the four musicians sawing away at cat gut, that the REF1000 sets the musicians further back in the venue and that the M300 brings them forward in a more intimate perspective, I justify it not by the certainty of my perceptions, but by the fact those perceptions have recurred over several months. Other than these qualifications, both amps do a splendid job preserving the dynamics, sonority and transcendent energy of this music, both amps facilitate that all-important connection. At the risk of paraphrasing St. Anselm, what goes on in the Late, and some of the Middle, quartets is that which is beyond conception, beyond words, and these amplifiers reveal this without intrusiveness."
A word of warning - these amps need a lot of break in time. Mine started sounding smoother after 100 hours and stopped improving perhaps around 500hrs. Don't make any rush judgment.
