CM, welcome. The rear port won't be a problem if a little "breathing" room is allowed, and the 3" you mention is adequate, assuming that you don't mean that there would be a tight seal at the front edges. The front would be a bit more of a problem(with any speaker) if you recess it into the entertainment center so that the sound diffracts(bounces off the edges of the e.c.)on the way out. It would be helpful if you could bring it out far enough so that it extended at least a fraction of an inch.
Don't know anything about the HTD speakers, but the front port has no advantage over the rear port, as described above, so go with the excellent M22s.
