T+A Class D Amps ( M200 or A200)?

Really depends on what specific improvements you’re looking for and what sound characteristics are most important to you if you could share that critical info.  There doesn’t look to be much by way of reputable reviews out there on these amps so a bit of an unknown.  For a Class D alternative I’d take a look at the AGD Audion monoblocks or Tempo di GaN stereo amps as they’re more well reviewed and more of a known quantity.  They’re also quite a bit cheaper, and they can be easily upgraded by the user when technology improvements are introduced (and they have been), which is very nice — not sure if that’s the case with the T+A amps.  AGD offers a 14-day return period less shipping and credit card fees. 

@soix, thanks for that advice. Both AGD and Tempo have escaped my radar. I would be happy to spend less!

To answer your question, the Characteristics I’d like are, very generally, to keep the neutrality of the M33 but if possible add a touch more oomph (for lack of a better descriptor), even though the M33 is 200 watts per channel at 8 ohms, there is something about the 6 ohm & 88dbs sensitivity Epicons that makes me feel like they could use some more power and I am purely guessing  that the higher voltage T+A s might create a bit more crispness across all frequencies, without moving into harshness or sterility. 

taking the NAD C399 that currently runs the Revels and using it just as a pre-amp/streamer/DAC (which it is pretty well suited to, despite lacking balanced outputs) and for a pair of T+A M200 mono blocks for the Epicons.

I didn’t read your post closely enough before I responded — my bad.  You do not want to pair the C399 in any way to a good amp as it’s a total mismatch from a performance perspective and will be a huge bottleneck.  

Thanks, @soix.

I had my concerns about that, but figured i might try it initially while i search for an alternative. I am also wrong about the nominal impedance of the Epicon 6s; it is 5 ohms not 6; even more of a concern about power/voltage.

You’ve got nice speakers and sounds like the M33 isn’t really doing it for you.  Reading between the lines it sounds like you’re feeling like the sound is a little anemic/thin sounding, lacks some life/emotion and could use more heft, weight, tonal density/color but you don’t want to sacrifice the detail in the treble while avoiding harshness/brightness.  Does that sound close to what you’re looking for?