I've just upgraded from the REL Q150E and I know that these are two completely different subs but the difference is unbelievable.
The Strata III is paired up with 5 Bowers & Wilkins M-1s. With the Q150E, to get a decent bass response (e.g. warmth in the sound and high impact bass for action scenes) it did have a tendency to make the main speakers sound cloudy and a little muffled at times. To avoid this you have to back off the levels but then you lost the impact. The Strata, on the other hand, is unbelievably controlled and adds zero colouration to the overall sound dynamics. Infact, at times you actually forget that it's on at all.
The Strata blends seamlessly with the main speakers, adding depth and weight to music and dialogue without compromising the overall quality. With action scenes it really come in to it's own. One minute all is calm and the next the room is shaking but all of this is incredibly controlled. The Strata is very quick and precise. There is no boominess to the sound that it produces. You can feel the lower frequencies but they do not linger, they're instant and direct unlike other subs that seem to try and overcompensate their weaknesses.
I do have one disagreement though and that is in the way that the manual suggests that yo set it up. It states "Phase Orientation: Keep in mind; the right phase is whichever position is the loudest or fullest." To achieve this the phase was set to '2 - LFE' but I found that setting it to 2 actually flooded the sound and by the time you turn the levels down to stop this it completely lost it's impact. Leaving the phase set to '1 - Line' didn't give the fullest sound on set-up but is much better for home cinema. You can adjust the levels back up again and not lose any impact or detail but maintain the balance of the sound.
Final thoughts - Fantastic sub that makes a huge difference to your viewing experience. If you've got room for one or a corner that you can hide it in you definitely won't regret it.