tom:

I'm not sure of anything yet, ha!.

I'm somewhat leery that the mild M3 bass bump at 100Hz (that sounds so good without a sub) combined with the reinforcement of the alcove plus the low end contribution from EP-400 might be too much. That perhaps could cause some bloated bass in that region.

The problem is, I cannot correct for that as my Tube Amp has no tone adjustments - only volume. That is why I'm leaning towards the M2s.

TAM

Another reason to go with M2 and sub will be the proximity to the wall. Too close to wall might bump the bass of the M3 even more. M2 won't have that problem.

Since you already have M3 I would go for M2, just for the fun in the variety. If the M2 doesn't work out, send them back and try again.