I have a pair of Sopra 3s that I tried to integrate with a pair of REL s/812s. I tried for months to get them to sound acceptable in my room, even broke out the REW software. But no matter what I did those RELs wouldn't integrate well enough.
Someone proposed the idea that I didn't have enough control over the RELs and that they wouldn't necessarily go deep enough anyway. They said try to audition a pair of Rythmik f12s instead because of the number of controls on their back plate.
Well after a few months I did manage to get a pair and I found them extremely easy to integrate into my stereo system. And it only took me a few days in total to get it right. I then gave back the Rythmik f12s, sold off my REL S812s and bought a pair of Rythmik G22's.
Couldn't be happier... Although I'm thinking about buying one more pair of them.