I recently picked up a M12 and M22 used. For the system I’m using it in - movies & streaming - it’s fantastic, and I really enjoy it. When I tried them both in my #2 pure 2-ch rig (not the main 2ch but still nice): the M22 fared extremely well, paired with a VAC tube preamp and tube phono. It’s not an amp that will add sweetness, but it’s super dynamic, punchy, and powerful. The combination with upstream tubes & vinyl is great. I might actually "borrow" this amp once in a while for the 2ch, when I want a different sound.
The M12 didn’t fare nearly as well. It’s clearly "solid state" sounding (or is that "digital" sounding?). This unit can’t hack it in one of my pure 2ch systems. That said, I absolutely love the thing in my living room system for its convenience and rich feature set.
So while your M33 is newer and higher class than my M12 + M22, I think the pre section of your M33 (newer M12) might be what’s holding the amp (newer M22) back. And I think I understand why you might say this sounds "grey". Unfortunately, I don’t think the pre section’s influence is bypassable in your case - it converts all inputs, even analog, to digital and runs it through the processor and DAC :(