The best way to describe the sound of TBIs is that you don't hear them at all. What I mean here is that there is simply no overhang, the notes stop as quickly as they begin, with excellent detail which is obscured and blended into one mush by most other subs. What these amazing little subs do is create the illusion that the main speakers are simply reaching lower - the integration is very seamless. The Rel Stentor was very similar, however I could hear a little bit of thickening and smearing, which I don't hear with the TBIs. The REL would be a better choice for larger rooms as the TBIs are obviously limited in the amount of air they can pump.
One thing I should note: although the stock amp is decent, I am running my subs with a pair Acoustic Reality ICE based 600 wpc monoblocks and I'm using a high quality external crossover set at 55 Hz with 18 dB slope.