I forgot to ask -- how wide is the "slightly narrower section" where the screen and front speakers are located ?
Looks like run-of-the-mill foam is not going to be effective in the 150 Hz range -- you need something more like a RealTrap (membrane trap) to work in the lower range. Peversely enough, the RealTraps SoffitTrap has an absorbtion peak right where you need it, but it's exactly the wrong shape to stick behind the M60s. Personally I would just temporarily pull the speakers out a foot, run a quick graph to satisfy myself that the 150 Hz suckout moved to the left, and then forget about it and watch some movies
