I have two LH 220 vacs in a modified black box (I built it for 2 motors) and am getting 8hg with both on. I haven't sealed my bleeder board down yet but on one zone (2' x 2') I threw on a piece of 1/2" mdf just big enough to cover the zone and I couldn't move a 12"28" piece of hickory. Hope that helps, I plan on trying to put down a full sheet of 3/4" mdf this weekend and will know more then. I really hope I did everything correctly and it works but it looks like it will. I will update this thread when I have completed it.

Joe