I've had the M2x for a while now and really enjoy it. Unfortunately I don't have the M3s to compare directly, but do you have experience with any other DAPs? What I can say about the M2x is that you definitely have to go 2.5mm BAL, for some odd reason the BAL isn't significantly more efficient but definitely sounds better. Without getting into that whole debate, let's just say I suspect the device feels like it's built around the BAL output. That's not to say the SE isn't good. It is, above average I would say - think Cayin N5ii but warmer more natural (I hope I'm using those adjectives correctly).