Polk Monitors

are efficient so a 5.1 receiver with 75W per ch + powered sub will rock your cave. In this season, you should find some mid range receivers on sale for under $250. You missed some under $200 on BF but those prices may come back before 12/25.

If you can be patient, wait for Newegg to put Polk 10" powered sub and Monitor 30 or 40 surrounds on sale.

Good luck.