Hi JohnK,
I have exactly the same query as TylerC and for the last 6 months, I've been scouting my area for a small sub to go my M3s. I had the opportunity to demo the B&W AS-1 and the Onkyo SW-105. Surprisingly, I found the Onkyo to have clean and tight bass as compared to the AS-1. The AS-1 has only a single choice for Music or Home Theater and a bass level. No adjustable crossover and the bass was boomy and unclear. The SW-105 was able to disappear into the room very well.
But I'm still looking for other subs to demo. You mentioned good small subs in the $200 range. I'd be happy if you could point me to some of them. I'm also looking at the DIY option.
