I acn never remember who makes em... Steel City I think. (TB?)
Fully adjustable round floor rated outlet box. 2 1/2" high extra wide box, with an adjustable brass ring that comes up to the finished floor. Then a heavy brass cover. It's one of the few types we can use here.
Anyway, I usually put them in concrete, but it sound like you may be able to put one between the sub, and finish floor?
I found 'em! Warning these things are not cheap! http://elec-cat.tnb.com/tnbcat/tnbcat/SCFB_home_page_dsp.build_catalog_home_page
Go to "floor boxes" - "Stamped Steel Floor Box Line for Use in Wood Floors"
There is a picture of excatly what you want to do! ( I think? )