May I ask what amp and speakers do you use with M66? Did you also enable dirac? Why would you say M33 be better? Sound characteristics are different?
Initially i thought of trying M66 but mostly due to Dirac Live built-in. Now I am hesitating. I also liked the modularity idea, where I could start upgrading the equipment before moving A500s completely to another room - get M66, enable Dirac, connect with my A500s via XLR, and after some time decide what amp + speakers I want to put.
Currently I use the M66 with a pair of Atma-Sphere Class D mono amps. Very good they are too, from a long-standing and respected US valve amp builder who has seen the light and started building Class D amps using Gallium Nitride FETs. These drive Avantgarde Duo XD speakers. The speakers are a very easy load being 107 dB sensitivity and 18 ohm impedance. 100 watts is more than enough power.
I'd been using the M33 for 3 years or so and loved it. The M66 seems to be an upgrade, but it has lots of features missing compared with the M33, although these are generally not sound quality related. I was mentioning the new M33 Version 2 that seems to offer even better performance than the M66, in terms of DAC anyway. The V2 features DL and DLBC (same as M66) but caters for only 2 subs not 4 as in the M66.
However, if you have genuinely full-range main speakers as I do, all these newer NADs have a serious problem if used with subs. In the Settings, you have to state there are subs present (otherwise no signal from Sub Outs) and you have to specify a crossover that governs both the main and sub speakers. In my case, if I select 2 Subs and choose say 70 Hz, my main speakers and crippled as they are deprived of those low frequencies and I have to ask "Why spend so much money on full-range speakers for them to be unable to deliver deep bass? It's odd and very distressing as earlier NAD Master Series (M12 and M32) allowed different or no XOs to be applied to Main and Sub outputs.
So, with my system, I tell the NAD that there are no subs, but I feed the subs directly from the speaker output of the power amps - or via the preamp outputs if I want to use low level signal. This of course messes up the abilities of DL and DLBC as these DSPs will only recognise 2 speakers - left and right – ie the combined output of left main plus left sub and right main plus right sub.
Are you following all this? You did ask! Happy to explain further if needed.
PS – If your main speakers are not full-range, the compulsory XO is likely to be beneficial when subs are added. You will protect your main speakers from deep bass so louder levels are less likely to result in distortion.