Best bang for your buck would be either:
Another sa-15 so you have better efficiency and can wire down to 1 ohm on your amp (if you're almost satisfied with 1 sa-15, 2 with lower impedance will more than please you)
HCD4 18
Fi SP4 18 (doesn't need too big of a box, that's a plus)
21 inch Death Penalty here in the classifieds. It will be so freaking loud... Needs a huge box though.
With either of the above options, you'll be quite loud with the amp you have but if you want to get ridiculous and actually get those subs to do what they're made to do, it's only like $1000 more for an alt and a ~4kw Brazilian amp, and maybe a battery... Tune low 30's. 28 at the very lowest if all you mostly listen to low bass but it'll decrease output noticeably for a practically imperceptible improvement in low end extension. For the HDC4, tune 33-36hz, though, they don't like low tuning. And the sa's don't like below 32. For the Fi and Death Penalty, 32hz would be a good place to start but you can go slightly lower.
Based on a ton of research and some experience.
