>Would you replace the FX300's with the RTi4's and go with the R15's for the back channels?
Is the fx300 the older version of the fxi3? If so, is it dipolar? or bipolar? or whatever they call it? I haven't experimented with anything but direct radiating speakers, like the rti series, so if the fx300's are di/bipolar, then I can't tell you which to use. Some folks claim the additional drivers add depth to the soundstage.
About the r15's, like I said, I'm not real particular about my 6th and 7th channels, so I'd say use "whatever" since so little sound gets sent back there. It complements the other 5, but never seems to be a big deal that I use my really "bad" speakers there. If you get a chance to listen to the rti's, r15's, and the f300's, that's your best bet. Then you can make the call based on position in the system, price, and your preference on the sound.