Well, I can only speak for myself but since I don't know what any sub will do in my room, I will likely go for overkill and just turn down the volume. It is better to have more power to call on when you need it than not enough. With that said, I'm not sure if a few Rythmik 15 subs would be enough for my room but then again I have a feeling they might make my ears bleed. Based on reviews I know Rythmiks sound good and I know they do great for music. But for shake the room bass, maybe they will, maybe they won't as there aren't enough reviews of multiples of their subs out there for strong theater use. And when I see people replacing their Submersives and LMS5400, etc with multiple of the DIY SI18 and being happy with the sound, I have to take notice. Part of my concern comes from Rythmik themselves when I see comments like they don't try to cater to certain audiences, and when I read that I think the ones that wants lots of home theater bass so that makes me wonder if he will even do an 18".
All that said, an 18" Rythmik would be great and a full double amp in the same heatsink to do dual-opposed would be even better. Based on the research I have done, I think there is a market there with a little coaxing, primarily by just making sure the Rythmik subs get to some of the sub get togethers like Mark Seaton does with his Submersive and Jeff does with his JTR. Actually, if Rythmik would take a couple of their FV15HP subs to one of the sub GTGs they would probably get more purchases from the home theater crowd, especially with the DIY version available.