I'll give you an opposite viewpoint. If you can't afford a good quality sub, you're better off without one. I've tried every iteration from 2.0 to my present 5.2. In fact, my two Celestion 8" replaced (and bettered) a cheap 12" Polk boomer, yet they're still borderline and sometimes I watch movies with them off, it depends on the movie.
I would say entry level would be something like a Rythmik LV12R or SVS PB-2000. Otherwise I think you'll be detracting from what you have, particularly with 4 X 8" drivers in your main speakers.
From my experience ranging from 2.0 to 5.2, for overall HT movie enjoyment I would add a center channel speaker before a sub and I would add surround speakers before a sub. This is of course with a multi-channel processor and amplification processing actual Dolby/DTS soundtracks, which I don't think you have.
I'm probably in the minority, but I've experienced too many years of my own and friend's unnatural LFE sound effects provided by marginal subwoofers.