t2d2

iTrader: (8)

Not sure if links are allowed, but search Crutchfield for MTX TM1004. It's a 10" marine subwoofer (should make it very robust) designed for free air use and having a 4.39" top-mount depth, so it should be an excellent fit for the SC300/400's OEM sub mount on the rear deck. The price and specs seem pretty reasonable, too, although its low end isn't super low.

t2d2

iTrader: (8)

I found another option worth considering: MB Quart NLP254S 10" marine sub with sub-3" (!) mounting depth, and designed for free air use. 300W RMS and supposedly goes down to 25 Hz, but I'm not sure if MB Quart's reputation supports those specs as being legit.