If you want to retain the stock amp, I would just grab a quality woofer form some place like parts-express
. They sell all sorts of stuff, but what I love is their selection of speakers - all sorts of sizes, manufacturer and impedance.
You've probably not heard of most brands they sell, but there is some quality product to be had. I replaced the driver in my Velodyne home theater subwoofer with a Dayton (went from a 6" to an 8") and the sound is impressive for a ~15 year old Velodyne.
Anyway, I'll have to do some research, as without the impedance of the stock sub, it's hard to start to recommend much. However, it's probably a 4-ohm sub from the factory (I would think?), and I doubt the amp does more than 100 watts RMS at that 4-ohms.
That being said, something like this
would probably do nicely. If it's and 8-ohm setup from the factory, there's an equivalent Dayton to fit it.
I'd have to dig out my stock SVT sub box and check the sub (impedance, mounting depth and such) to be sure of my recommendations, but for ~$60, the Dayton stuff is hard to beat. The one that I put in my Velodyne home theater sub was the ~$25 one, and it is seriously impressive (I did also replace the passive radiator on the sub, as the original paper one was bottoming out, but still, for less than $50, it's a solid rebuild).
From my experience, most of the "car audio" company's 8" subs are usually lack-luster at best. And they usually seem to be astronomically priced, given their small size.
Just my $0.02
If the link to the sub I suggested doesn't work, just go to parts-express
and search for 'RS225-4.' For some reason, the direct link doesn't seem to work.