It depends on what you are trying to accomplish. They will sound great using SACD or DVD-audio as a source.
They will be a little less effective for home theatre. The driver on the 6 is actually woofer size (generally accepted to be 6 inches or larger) and will produce great bass and moderate mid-range.
The problem is that bass is omnidirectional, while midrange tones are much more directional. The bass you have in your primes and sub is more than enough for your application. You don;t really need any more.
The midrange tones are very effective for home theatre because they spread the sound around. It carries the motion with it. The whole idea behind surround sound is for the sound to be coming specifically from a certain point (whether behind you or in front) to match the action on the screen. A 5 or 5 1/2 inch midrange will be a much more specific sound source than the 6 1/2 woofer. If you really want dynamic sound FOR HT, then go with a smaller unit than the RT6.
The strange ones you saw on the Polk website were probably the triangular FX-series. They are designed with two inputs on the same speaker for a bipole/dipole set-up. The technogaga on bipole/dipole is best left to others from this site, but simply put, they are optimized for SACD and DVD-audio.
The RT4 shelfs or even the Atrium series (which are really easy to mount) will do just as good a job for most applications.
Good luck with your soon to be new toys.