I can't speak to the Mac since I don't own that amp but I've had the PS Audio S300, M700s, and M1200s and have used them all with the X-LS Encores. All are excellent with the Encores. In addition to the Encores, I have used all three amps with the NX-Otica MTMs which are much more sensitive than the Encores.
While all three are very good amps, at the last Lone Star Audiofest both the X-LS Encores and NX-Otica MTMs in my room were powered by the S300 and considered excellent, moving up the line brings even better performance. Moving from the S300 stereo amp to the M700 monoblocks was a big step up in performance. Back and forth comparisons weren't needed. I did let the M700s burn in for about 50 hours before comparing them to the S300. About three months later I got the chance to be in on the hybrid (tube input stage/SS output stage) M1200 beta testing. I went into the beta testing intending to give them a workout and provide my feedback but not intending to keep them. For that to happen the M1200s would have to be better than the M700s and, when I was in the mood for tubes, my PrimaLuna Prologue 4 with NOS tubes. Right out of the box it was evident the M1200s were up to the challenge. Like the M700s they took some time to burn in, about 200 hours with the M1200s before they stopped improving, but within two days the PrimaLuna was boxed up and traded in on the M1200s.
With the PS Audio Stellar amps, let your budget decide. At their respective price points any of the three amps will provide a very enjoyable listening experience. That said, if you have the budget for the M1200s, get those.
Mike
