The StudioMonitor 350 are bookshelf speakers with 1" tweeter, 5.25" woofer and 8" passive radiator. That passive radiator really works quite well, giving the speakers great bass extension, but my only gripe is that it's a bit weak. That's why I was considering adding subwoofer.
I get what you're saying though, 6" is just way too small to be a particularly good addition to what I already have. With that in mind, how big is too big? I'm mostly looking for a subwoofer for music listening, not home theater. I've seen BIC and Klipsh subs on sale every now and then, but wasn't convinced something meant for home theater would be ideal for just music.