Dude, I was in the same boat you are in when I first started looking at subs for my speakers. I didn't want to spend more than $300 tops. Luckily, I started doing my research like you are now instead of making an impulse buy; reading these forums and asking the same questions. I soon realized that $300 wasn't going to buy me much and it would have been $300 of disappointment. I decided to wait a little longer and go for the SVS PB-1000. And while the sound was great and it dug down low, it didn't have the output I wanted so I sent it back and got a PC12-NSD instead. I was very happy with my decision and did not feel that I 'spent too much' on the sub after hearing it...it makes that much of a difference! Plus, I was lucky to go with SVS and send back the PB-1000 at their cost and pick up the NSD instead.
So bottom line, it does not make any sense at all to go 'bottom of the barrel' for a sub with those speakers and AVR. A good sub makes ALL the difference in your HT. It is such a crucial piece of equipment Wait a little bit longer, save your money, do more research, and get a PB-1000 at the minimum! Currently, I'm using SVS upgrade policy to go for either a PC12+ or dual SB-2000's. The sub-bug will get you...be warned lol