I believe it's been posted a few times throughout the thread that connecting more than one headphones at a time does in fact degrade the sound.
However, since I don't own an M9 myself, I haven't had a listen to see. Now that you've got KingWa's blessing, aka it won't blow up or fry something, I'd just sit down and listen. Maybe you notice a difference, maybe you don't :-)
As for me, I'm still hoping to tackle my Prickely Peete Phoenix MAXX Mod soon and see how it sounds compared to default AND see if I can put it in the same room as an M9 for comparison. I've had high hopes for the M9 ever since I read that it seems to have more musicality or emotion than the M5/6/8.
Any further feedback on the M9 now that a bunch of you have owned units for a while and the sound should have settled in? And I mean both the amp itself and you guys to the sound of the amp.