I have tried the Hegel 390 with my B&W's/Harbeths and use a REL T7i to fill out the bottom frequencies. Works well. I have heard the Rel T9i connected to a similar setup at a friends..and they are a little too much bass for my liking for two channel. I have also heard the Rel S5/S510 in a bigger room used with Hegel's and Focal's...huge soundstage and you need a big room. I keep my subs at a level where the bass is not too boomy...almost like you would not know that a sub is in there. I have no experience with Rhythmic or SVS subs.
Jit