I've got mine set up at c2 which is at 49 htz. Our Kef's is set for 55 htz. I am still tweaking between C5-C2 but for the moment it seems like C2 gives me the best combination
I find adjusting the gain control of the sub is critical. I initially had the gain control turned up pretty high so I can "hear/feel" the bass but eventually, depending on the music, it did not sound right. (Too much bass made the system sound muiddy) So i brought the gain down quite a bit. Matching the gain level of the REL with that of the speakers made for a better sounding set up.
I am assuming your gain is turned up quite high to get the slam from the Home Theater movies. You may want to bring it down when you listen to stereo only. Unless your REL has dual inuts? One for stereo and the other for Home Theater? If so you can set it up so the gain in the home theater is higher than that of the Stereo input.
Regarding placement, my REL is not placed in the corner of the room. It is in the middle of my two speakers. Why is it there you might ask. Because I have a narrow litening room and if I put the sub in the corner my speakers will end up being closer together giveing me a not so ideal sound stage. So I figured that I will put the REL where it would best suit my set up and tweak the unit for optimal performance. Afterall, the REL was created to be tweaked .
I hope this helps. Good luck on your tweaking :)