Let's just say I wouldn't be holding my breath. Cans like that are pretty noncritical. They're generally fine with around 2 Vrms max output and <25 ohms of output impedance, plus easy on the output stage at 300 ohms. (Keep in mind they're rated 97 dB SPL @ 1 mW. You need all but 4 mW for 103 dB and 20 mW for 110 dB SPL. That's 2.45 Vrms = +10 dBu. At the same time, they're not too fussed about 10-20 µV of output noise.) The only thing even less critical I can think of would be an ATH-R70x, similar sensitivity but at 470 ohms nominal.
So generally speaking, HD600s are among the last cans you need more than a
basic amplifier for. Something MAX97220 based is just fine. The M2's output ought to be perfectly adequate, it measured well in
Julian Krause's testing. The only thing one
might need an extra amplifier for is more gain for material with unusually low digital levels.
It is always easy to be tempted by better performance and higher output and whatnot, but at some point output impedance is low enough for a <0.3 dB deviation in frequency response, maximum output is loud enough and noise and distortion are inaudible. If you get to this point, the gear is
transparent. A better amplifier may achieve this over a wider range of loads (you might have both super sensitive BA IEMs and an AKG K340 or a Hifiman HE-6 in your arsenal, now that gets tricky), but with a lone HD600 you should find that a lot of solutions are 100% equivalent, with differences in performance being measurable but not audible.
In the upgradeitis-infested lands of audiophilia, there is never such a thing as
good enough, of course.
For some real audible changes, I might give parametric EQ a shot; you'll find some presets for the HD600s out there.
