Using a Blueray 7.1 rca out and power amp. is a good idea.
But what about volume controle ?"
Volume control is adjusted by the Bluray player remote.
This setup works very well. I connect my hd Xfinity cable via HDMI from the cable box to my Oppo 105's HDMI input, which gets the video and audio to the Bluray player. Then the video is connected to my hdtv via HDMI.
For audio, I run xlr cables from the OPPO's L+R stereo outputs to a pair of class D mono-block amps that drive my main Magnepan 2.7QR speakers. All the surround channel outputs for up to 7.1 on the Oppo are also connected directly to the amps for my 5.4 setup via standard rca cables(the center channel to a class D stereo amp bridged to mono for my Magnepan CC3 center, the two rear surrounds to a stereo class D amp and the sub output to a 1K watt class AB amp/controller that powers a 4-sub Audio Kinesis Swarm distributed bass array system and controls volume, crossover frequency and phase on all four subs).
There is some audio setup required on the Oppo's setup menus, all done from the remote and displayed on the hdtv, such as setting the crossover frequency for the sub output, setting the Size of each speaker channel('small' for sub assistance and 'large' for no sub assistance), Distance(set in feet from the primary listening seat) and Volume or Trim Level( relative volume of each surround channel set sequentially by ear or using a sound level meter at the listening seat.