Eandylee, the REL makes a very good product. I occasionally use a Strata III with my Stax F81 electrostats with good results. To answer your question, I use that combination because I also have a pair (actually, two) of full-range speakers, but mainly because there is nothing I have heard that would make me give up the midrange magic and sound staging of my Stax. However, sometimes their very limited bottom end is too hard to take, so I augment it with the REL. Integration is good, but not perfect by any means. Keep in mind that integration with electrostats is going to be problematic under the best of circumstances; but it is possible to get integration at an acceptable (for me) level. Additionally, the sound staging is further improved by the addition of the sub.
Having said all that, a few things to keep in mind:
-In my case the best results (by far) are achieved using the high-level inputs.
-I don't know what SPL's you expect, but don't expect miracles. There are other subs with much superior power handling.
-The quality of the bass is very good, but I still find it to be a little on the
soft side. Keep in mind that I use it with electrostats.
-In spite of what even Sumiko will tell you, one can improve the quality of the sub's sound by using a good power cord, and better cable instead of the flimsy supplied connecting cable; particularly the power cord.