So the best bet of improving sound is with better speakers, better room treatment or room correction (or even better, a combination of all three). Since the unit already has Dirac, I assume you actually use it already ? If not, do! It’s quite configurable, so you can tailor the sound the way you like it.
Isn't there a contradiction here?
If you have a good amp that delivers a nice flat frequency response and good speakers that produce a response-flat sound, there's no need to bugger up the amp's nice flat output because your room isn't perfect. You rightly say, put money into room treatment - this should obviate the need to mess with the lovely flat response your carefully chosen and costly amp was built to deliver.
I have the M33 but it sounds better without messing with the signal - because I've spent a bit of time and effort with my room - and a LOT of dosh on my speakers. and time setting them up. No Dirac for me thanks and I'll try to explain why I think Dirac and other DSPs built into full range amps can do more harm than good.
The signal (all of it) has to pass through this Digital Signal Processor. Remember those works - the signal is being processed and we all know that the less processing of a signal, the better the sound. Granted Dirac (in the M33) only ADJUSTS sub 500 Hz but the ENTIRE signal has to pass though the processor - there is no bypass for higher frequencies. This processing can slightly spoil the top end in a high quality system - a bit of the sparkle or goosebump factor is lost if Dirac is engaged. The only way to protect the top end is to have an active system with separate amps for bass and the rest, with DSP only in the bass amp. This allows the unmolested top end to continue sans-DSP to the mid and top drivers.
DSP has its place but only really in multi-speaker AV systems where the owner hasn't the skills to set up his complex system properly. Otherwise good speakers, properly set up in a reasonably well treated room is all that's required.
But returning to the OP's question - I agree it's daft to introduce an external DAC as it's analogue output will be converted back to digital and them to analogue a second time within the M33. A different situation with the new NAD M66 as analogue inputs can be switched to bypass the ADC and DAC, but this means Dirac's DSP is bypassed too.