For reference or for anyone interested, this is what I have come up with so far:
I have had to trim a bit more on the left side because the one seat bracket angles towards the passenger side, but there is still enough room for one or two of the slim subwoofers from Pioneer or Earthquake, as well as amplifiers and switching equipment.* The seats fold down most of the way, but are angled slightly.* It is not much of a problem, though.* If I just had a subwoofer and an amplifier, I could keep it low enough for the seats to fold fully, but I have too much to fit that way.