Sounds like it's gonna be nice.
What kind of money have you budgeted for speakers and amplification?
The only drawback to flush mounting the sub is you will be pretty much stuck with the location of it and it's hard to test it before hand. It's not like you can go around cutting a bunch of holes in the wall to test it in different locations. It also limits you to a forward facing sub... no downfiring. Of course you could do an IB if you could work it out either in the Store room or Electrical room on that side of your HT room.... or the attic.
Good selection on the Z4
