The one thing I'd be concerned with in using a sub for music with full-range towers (or perhaps any speaker) is finding the right blend, by way of x-over point chosen, sloping, phase etc. I briefly tried using a sub (when they were with my stereo mains), and I just settled on 2.0 stereo. You will want to x-over the sub, even with full-range mains, to avoid phase and amplitude issues.
For HT, I find that 80hz seems to be the ubiquitous solution for a x-over point.
Assuming I am right in the first paragraph above, how will you easily switch between the two settings? For instance, one pro reviewer that has my mains uses 28hz as x-over point for the sub in his music stereo system. I guess I just want to shed light on the whole BM (bass mgmt) thing. Some receivers, processors, are more powerful than others. I know the SMS-1 is supposed to be pretty powerful, but I do not know to what extent, and Im sure others have found more affordable solutions.
It could be you end up like me, and just forgo the sub for stereo use altogether. Well, they are no longer combined, and perhaps I would have tried again had I more flexible/powerful equipment. I sure wasn't going to fire up the display everytime I wanted to change x-over point in my receiver, just to get to the GUI!
edit: wanted to add.. a sub is pretty much a requirement in HT because the LFE is only meant for the sub. Some argue that some of this info may go to the mains, but I share the same present impression that the mains don't often get a whole lot of the "LFE material". Using a 2.0 system means you miss out on a huge amount, if not all, of the LFE.
I know a while ago when you started your speaker quest, that I already recommended JL Audio for a smaller music sub. I own ML products, and I still found the JL to be superior to Descent/Depth at a very similar street price point. Gorgeous too, and decidely smaller than most subs with comparable performance.
The Velo DD series is perhaps the other usual candidate, and I would consider them more strongly in a smaller room if only for their more powerful onboard EQ, but if you get an outboard BM system anyways...