Did you set the gain? That sub is 250w RMS, 500w max. Use a multimeter and set the gain like this:
You can go a little over 250 if your electrical can handle it. I would shoot for about low 300s though. This is where I would start. From here you know that sub is doing the most it will ever do.
1600w on the pioneer amp is "max rated". That sub is 4 ohm SVC, and at 4 ohm, that pioneer amp is 300w RMS. Use the pioneer amp for that sub, but don't expect much.
THEN if that is not enough, you can get a sub that can be wired to 1 ohm, @ that 800w it can do. I would look for a sub anywhere from 600-750w RMS. You could probably set it from 800-900w no problem, depending on what you get. It won't shake the windows but you'll feel it enough to help the sound quality. You can spend your money for this, or go big. If it's me, I'm probably looking on craigslist for a sub and enclosure, and this is if your electrical can handle it, and how deep do you want to dig into your wallet.
As for the fosgate amp, I would personally just call it a paper weight, or maybe you can sell it for a few bucks. It's just not worth it to me. At most you can play two channels at 2 ohms and get 75w each, at 4 ohms, a head unit will do that much, and it's not worth buying the splitters and extra electrical need just to run it. You'd have to buy really low wattage speakers, when 100-150 is a lot more common, and only about $100 for a 4x100 amp. The fosgate amp is nostalgia, maybe someone with an old little import that doesn't want much.
Personally, It may have seemed like a nice gesture, but it just seems like it's equipment you would pawn off on someone. I'd trade that for an extra head unit that was decent, or a couple pounds of venison. Maybe a small tool or something else that was useful, because this isn't something anyone would buy these days. My guess is just 10+ yr old equipment that they have upgraded since. Just saying don't expect much if you start spending money on other equipment to make this equipment work.