Hello PaulHa,
I do not recommend installing any high-quality, neutral speakers like Axioms into an entertainement center, because boundary effects from the surrounding cabinetry will indeed degrade sound quality--to what degree is unpredictable.
Designers go to considerable lengths to reduce edge cancellations and effects of a speaker's own front baffle and grille cover. Good speakers are designed, measured, and intended to operate more or less in free space, unencumbered by extra cabinetry. Of course, we all live in rooms (the other enclosure!) so a speaker should be positioned to avoid gross effects from nearby walls by allowing some space between the wall behind the speaker and any side walls.
But installing a speaker into an EC undoes all the good intentions of the designer by substituting new cabinetry that will generate reflections and cancellations that can't be controlled.
If you must do this, there are a few steps you can take to reduce boundary effects. On the doors in front of the speakers, use a plain grille cloth with no decorative wooden or plastic latice work. Remove the M60's own grille and move the speaker as close as possible to the grille cloth of the EC's door. If it protrudes somewhat, even better. Use it without the doors if you can convince your spouse that it will sound much better (and it will. . .). Try an A/B test with one M60 out in free space and the other in the EC, using your receiver's balance control to go between one speaker and the other. If you hear dramatic differences in reproduction, with a "hollow" or colored sound from the speaker in the EC, you'll know how much you sacrifice in fidelity and transparency.
The subwoofer is the one speaker that you cannot plop into an EC and expect it to work properly. Its performance is very dependent on its location in the room relative to your listening area. A good place to start with a sub is putting it into a corner (any corner) and, if the bass is too boomy, gradually moving it out of the corner along one wall or the other until you get smooth, deep bass heard from several seating locations in the room.
Regards,
