I purchased a pair of 6Ts and an Onkyo 606 about one month ago.
One surprising observation was that after Audyssey calibration, switching between "Stereo" and "Direct" modes made a very large difference in the sound -- Direct basically disables Audyssey EQ. So I assumed that either the 6T had nowhere near flat in-room response in my room, or Audyssey calibration was totally inaccurate. It turns out that I had "Dynamic EQ" on, which seems to have little or not effect in Direct mode but a huge effect in Stereo mode -- it boost low bass about 3x (subjectively). With Dynamic EQ off, the difference between Direct and Stereo is noticeable but quite subtle. I post this as a warning to others.
Overall I'm quite happy. I can't provide much an audiophile review, because there's nothing else here to A/B against, but...
Compared to my TV's built-in speakers, the most obvious improvements are in bass extension, lack of "boxiness", and stereo imaging, perhaps in part simply because the 6Ts are further apart than the TVs speakers. Pretty much every other quality is probably better as well, but those three are so obvious that it's hard to notice anything else. The difference is most noticeable on music, then movies, then TV.
The other product I can compare to is an old pair of Grado SR80 headphones. Even though both products have fairly flat frequency response graphs, I subjectively find the 6T to have more bass and less treble (in the range of cymbals and such). As a result, the 6Ts sound great on a wider range of material, while the Grados sting on bright recordings. (So if you're a happy Grado owner like me and thinking purchasing Aperions, don't worry about claims that they're bright.) I think most of the other differences just have to do with the fact that the 6Ts are speakers and the Grados are headphones. For example, slightly more detailed, analytical listening seems to be possible with the Grados, but certain stereo effects fail entirely on them.
I don't think a sub is needed with the 6Ts, but then I don't watch a lot of dinosaur movies. They look and sound really great, especially on music. They were easy to place, large sweet spot, really no problems. Although a glitch with Aperion's website dropped an item from my order, the overall experience has been great -- Jason Hicks was very helpful in resolving the issue and giving miscellaneous advice.
My only complaint with the 606/6T combo is that there's no way to boost the center channel volume (that I know of). On surround (e.g. DVD) sources, Onkyo's "Theater-Dimensional" mode fakes surround on just two speakers pretty well, but the level of the center relative to everything else is fixed. Most of the time the relative levels are correct, but late at night it would be nice to turn the sound down while keeping voices (which usually come through the center channel) audibly loud. Onkyo's "late night" mode seems to just reduce dynamic range (the difference between soft and loud sections). My current work-around is closed captions. Another work-around would be to buy a center channel and crank it up -- not sure how well that works in practice. Any other ideas?