While I had the Mazda, I had a plan in place to cram four sealed 10" Rockford P3S shallow subs (0.5 cu ft each) across the forward part of the trunk, set forward as far as possible to retain the max amount of trunk space. There would have been 2 mirrored boxes which would have spanned the entire trunk space side-to-side, and top-to-bottom, all 4 driven by an Alpine 1000.1 PDX amp. If I can manage it, I want to do that exact same setup to the Mark I get, or as close as possible. In my experience you can't get a better combination of quality AND quantity, within reason. (Short of a couple JL W7s anyways.)
I don't suppose anyone has the 2nd gen's trunk dimensions handy for sub boxes, do they?