I think the EP400 would add to the M60 and adds a little to the M80s, I could tell when the 400 was on or off with the M80s, it definitely added the lowest frequencies and sounded great, but with it's cost I would be hard pressed to buy one for either, especially the M80s, YMMV.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70