Thanks Georgestrings
anybody else????????
Basically, +1 to George. To add a bit more (I've owned the MPulse600 and have had quite a bit of time with the M6), I describe the M6 as the 'Mesa for those who don't like Mesa'. As you know from owning one, it is very hi fi, VERY aggressive in the uppper mids and treble, very little 'tube compexity, and it has a super wide, deep, low end. Very 'modern' slamming head.
The MPulse600 is among the most 'tubey' hybrids ever produced (IMO and IME), using 'tubey' as we often use it on TB to describe the very warm, fat, pillowy, low end, the nice harmonic complexity and warmth in the midrange, and the nice, organic, warm lower treble top end.
These two heads are on the opposite end of the tonal spectrum for me. Both ended up being a bit too extreme for me personally, but that is just an 'IMO/tone goal thing'.
I didn't feel the MP600 slammed quite as hard as the M6, but that could be partly due to the more relaxed voicing of the MPulse.
The MPulse600 will 'overdrive', but IMO that isn't really what it is about. Somewhat similar to the Genz Streamliner (although it has more low end extension, compressor, and more powerful EQ) it seems to be designed with a 'clean tube tone' in mind.
If you like it aggressive, deep, 'quick feeling' and solid state sounding, the M6 is pretty amazing. If you want it as fat and organic as you can get without buying a 75 pound all tube head, the MPulse600 is at or close to the top of the list of hybrids IMO.