Advice on 12" Sub

[esenior]

The subwoofer in my 69 Camaro convertible isn't performing as well as I had hoped. I'm not one who likes an excessive amount of bass, but my current setup is falling short and if I turn up the bass any more, the sub starts clipping.

Here's my setup:
AVIC X-930BT Head Unit
TC Sounds Epic 12" DVC Subwoofer (in a professionally built sub box, see picture below)
Pioneer GM-D9500F 4-Channel Digital Amp (for Fronts/Rear)
Pioneer GM-D8500M Mono Digital Amp (for Sub)
Focal 165 VRS Front Mids and Tweeters
Focal Rear Speakers (Can't remember which model)



Based on this configuration, and the fact that I have a rather LOUD exhaust, what would you recommend for a 12" sub?

Here's the current sub in the trunk: