I can't say anything about the MRE 220 monoblocks, but I have the RE 320 stereo amp with Super Black Box and couldn't be happier. I have 8 Ohm monitors, and the amp behaves very linearly on my speakers. Superior resolution of tone colors and timbral micro-detail, great separation of instruments and transparency, fantastic dynamics, transient energy and rhythmic drive. No 'warm' sound, great high frequency extension. Quite neutral as far as I can tell, with neutral meant in the best sense of the word, more like natural.
Interestingly, many reviewers have stated that unlike many tube amps the Octave amps can drive diffcult loads, with the Super Black Box connected, including Magnepans. But obviously, it does depend on the speaker.
A friend has heard te MRE 220 monoblocks on Magico M3 speakers and thought they sounded great. A bit richer in tonal color than the Spectral DMA-500 (or 400?) that he also heard in the same session, but apparently no anomaly of frequency response that would have been sticking out.