I'd love it if you could take a photo of your Rel Sub with the rest of your system, to get an idea how it correlates in size! Let me know, if that's possible! I'm thinking of maybe adding a sub that enhances the lower registers without taking anything away from the standmounts, but am trying to weight that against just getting some floorstanders like the PMC fb+ - I know the floorstanders prob won't have the same level of bass as a dedicated subwoofer, but since they might improve other aspects of the sound that the addition of a subwoofer can't, I don't know which option to go for.
By the way, what amps have connections to use a subwoofer with them?