Lee, when you say that I like the RCFs, you didn't mention which series, they make about 4 series of speakers, I'm not a fan of the lower series, but the top 2 series sound great.

I think my overall favorite would be the FBTs , but they are not cheap, but sound amazing. I think the FBT and RCF subs sound great.

I can look into the QSC subs if you like. Also another contender has been the new Yamaha series of DSR wood box speakers, with a 2" diaphragm horn, they sound great

The acoustic coupling effect is true on subs, I would always stack them side by side, instead of on-top of each other, once you reach a certain height, you will create a standing wave from the higher sub when the wave hits the floor, it will actually start to cancel out the lower subs waveform.

If you take your subs and always try to put them against a wall that will also increase the "coupling effect" and also help in making sure you don't accidentally create another standing wave from the space between the back wall and the subs.