always amp your speakers!!! in my experience more power equates to better sound, even if you dont use it. the amp isn't struggling to send power to your speakers so it will sound better at lower volume. when you turn it up, it isn't as labored as the stock sony amp inside the receiver and it will sound better. also, the sony amp will clip the signal when you try to get more voltage from the amp than it can produce but that is the same for any amp, some amps just put out more voltage than others.
if you want to go the easiest and cheapest route i'd recommend a five channel amp for those speakers and call it a day.
if you want to get (a little) more technical get a new sub as stock is only a 6.5 inch powered by 80 wrms. check parts-express.com for raw drivers (subs and speakers) and look for high efficiency speakers. the ratings are usually like 87db at one meter with one watt of power applied. high efficiency speakers will have ratings of 92 db+ and because how we perceive sound, a gain of 6 to 10 db is effectively twice as loud. another interesting acoustic fact is that a doubling of power yields a 3 db gain so you would have to triple or quadruple the power to a speaker in order for it to sound twice as loud
. you could use the stock enclosure, swap the driver and plate amp and have like 300 watts rms going to a 10" sub or go crazy and do a dual 15 dipole design. i guess it really comes down to how loud you want it, how much money you have and how much work you want to do. check diyaudio.com if you want to get into this stuff. the forums have a crazy amount of info on them if you sit, read and study for a few hours.
if it were my party rig and money weren't a factor, id scrap the receiver and play my music of a pc running j play, and a high quality dac then maybe some mini dsps to set my xovers then my own amps running at least 10kwrms. get 4 18s for sub duty, then 4 12s for 120hz to say... 3000 hz then a few horns for 3000 to 20khz. it would only be two channel plus sub and i'd probably do an ib (infinite baffle) sub install. all in all, educate yourself and you can decide what is best for your budget and tastes. no one knows you better than you and everyone has a different "best"
sorry for rambling, if you can understand this it should probably help haha good luck!! it looks like you'll need it.