Like many others I've jumped on the multiple sub concept/bandwagon. A single sub is hard to integrate properly. In fact, I never like the sat/sub concept. I could always hear the sub, unless it was run so low that it was essentially useless.
Lot's of post about this that can explain the theory better than me.
I'm now running a pair of GR/Rythmik OB subs that are essentially the woofers of my Merlin monitors, more than subs in the traditional sense.(I'm running them up around 80hz, which would be crazy w/ a single sub.)
An old Mirage sub chugs away quietly in a corner helping to smooth things out and make it blend better.
I think I now have a speaker system that can compete with many very expensive fullrange floorstanders.
I've not heard them, but that Usher has a very good reputation. I think you have the makings of a nice system going.