If you're looking for dipole surround speakers, I don't have any idea. But, if you're looking for a set, you can do one of two things.
You can try buying a full set (L,C,R,SL,SR) from a specific brand. The advantage is that the speakers will be timbre-matched, will have its phase correctly aligned, and will sound the same when a rotating sound is performed. For HT, Def Tech is a good one, especially since you already have 2 rear ones, you can spend more on the front, say the 2000 or even shell a bit more and get the 3000. Polk Audio although a bit more low-fi, is quite good for movies. NHT is another candidate, since they base their HT speakers from their audio lines. The drawback of using HT-oriented speakers is that they don't sound too well for stereo music listening.
The other way is to try to find a pair of good sounding musical speakers that you like, and buy 5 piece of them. This WILL be expensive, and another drawback is SOME of the more musical speakers aren't as dynamic as the HT-oriented ones. Since you have a sub, you can opt for mini-monitors, which will sound quite nice. DynAudio are good speakers, Martin-Logans are good for both music and HT, but require LOTS of power to open up, Magnepans are also good (again, lots of power needed), B&W Nautilus (IF you can spend the money).
Personally, (I repeat: PERSONALLY) I believe speakers are the most important audio link. Amps, and sources CAN affect the sound, but not as much as speakers do. I don't believe in cables. As such, usually for auditioning, I'll bring a familiar test CD, and try to listen to it and see the weakness/strength of the music being played back. You want to hear it spot on (inside the sweet spot), to the left, right, front and back, and even to the OUTSIDE of the speaker area. Some dealers will allow you to bring you own amp, and hook it up to the speakers, some have more elaborate HT setup, so it's a hassle for them to do so. I once brought my DVD player for this purpose, so I know some dealers will do it.
All in all, your ears (and your spouse's) are the things that matter. Good luck.
Ciao.
Muljadi Budiman.