I own the DTR9.1, and only have good things to say about it.
Onkyo is the parent company of them all.
In looking at the web site I can only see cosmetic differences in them, the RDA7 has 150W power vs 120W for the other and I think the warranty in the RDA RDC are 5 yr vs 3 with Integra.
THe Integra Research web site is a little low on info. I can say that when I have called to talk with them they have been very good to deal with, when I had the upgrade done.
THis probably wasn't much help, but if you are looking to buy, I went through the same questions, either get the DTR9.1 or go the RDA and RDC7 route.
I took the DTR9.1 it has the power I need for now, and later I can hook an amp to it.
Tony