I did SI-760 L/C/R and MC6-DB for the surrounds, and they work well together. RBH recommended that as well, and said there should be any problems using a different flavor of RBH for the surrounds.
I'd sell you any combination you wished to pay for also. I am also merely talking from personal experience, figuring out a lot of different RBH combo's for home theatres I find that MC-6DB's just don't sound that much different than MC-6's for rears. They were only designed/added to the MC series, to work as surrounds when 7.1 systems with B/D wall speakers started getting more popular.
As I explained before, both ways will work though, whether mixed with MC's or with Signature bookshelf models, Sigs will always give you optimum sound though as they do use a different, proprietary cone and a silk dome tweeter.
You can't pretend for half a second that the MC-6 or 6DB is as perfectly matched as an 41-SE or 61-SE they use different drivers. The 41-SE is actually quite a bit less expensive than a pair of 6-D/B's and is better matched.
Unfortunately, the 6DB's just haven't sold well compared to other bi-dipole products on the market, due to the recessed tweets that pretty much turn it into an MC-6 with an extra tweeter instead of a true bi/di pole.
Not to knock them but I don't see RBH sticking with them for much longer, but you never know, it's no tlike they are bad or anything, just not really any different from MC-6's besides being a lot more expensive.
CAN you use them for rears? Why not? As long as you do't mind spending the extra cash. Will you actually notice a difference to the MC-6? I highly doubt it, unless you convince yourself you do.
In the last 50 or so MC packages our clients have installed, not one has used 6DB's, just too expensive for what they are (which is REALLY odd for RBH as they are usually the best value for money around).
In short, like I said before, "given a choice" the 41's will dance circles around the MC-6DB's and for less money (up to $200 less). I didn't say there weren't options, but I offered my opinion having heard them set up in both combinations, I do this daily too.
Less money, better product, it's a no brainer really.