I've heard of the Multisub concept before, and I guess I need to break old HT habits. I think I can find a way (with my receiver) to let the tops run full range. This will at least give me -6db @ 60hz. I know I can't have my cake and eat it too (great bass AND small enclosures) so this is one of the compromises I'll have to make. I could EQ a little more extension in - at the expense of headroom.
I'll post updates as I make progress on the project.