Use stereo amp and speakers with surround receiver

I have a stereo system that I'm happy with but would like to add a surround setup which utilizes the existing amp and stereo speakers as the left and right fronts. My amp is a Rogue Audio Sphinx, speakers are ML Odysseys and I'll be using a Harman Kardon 50wpc 5.1 receiver.

The receiver has coax and optical digital out connections but the amp has no digital input although it has 3 selectable RCA input pairs. I have all other speakers in place and ready to go but am having some difficulty figuring how best to include my fronts maintaining power from the stereo amp.

Because the center and rears will be powered by the receiver, I need to find the best way to include the stereo amp and existing speakers for the left and right Martin Logan speakers. I have an extra DAC and am considering using the digital out from the receiver to the DAC and from there to an RCA input on the Sphinx.

I assume that this connection will work but am concerned about coordinating volume control from two amplification sources. Any advice would be much appreciated.
Does the H-K have line level outputs for L/R? Does the Rogue
amp have a HT Bypass feature or input? That is what you need.
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Thanks. The receiver is a Harman Kardon AVR230 for which I don't have a manual. It appears, though, that it does have a coax preamplication digital out connections for each of the speakers. The stereo amplifier has only RCA inputs so I would use an extra DAC to receive the pre out signal from the front L/R speakers, using the RCA output from the DAC to connect to the stereo amplifier.

My thought is that this will work electronically but my big concern is still the coordination of volume control. I'm thinking that the pre out from the receiver will control the volume of the front speakers through the Rogue with its internal amp powering the surrounds and center. The HK, though is only 50 WPC and the Rogue is 100.

I would appreciate your thoughts and advice in this matter.
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Thanks, Bob, I was afraid of that but I almost exclusively listen to stereo, including for tv and most movies anyway. What I think I'll do, then, to keep it simple, is operate 2 setups from the same system. I prefer the Jamo Concert 8's for the music that I mostly listen to so I'll connect them to the Rogue. For 5 channel use I'll connect the ML's as the L/R fronts and the center, sub and rears to the HK.

Your comments re the unity gain on the Rogue, if I understand what that means, is the primary reason that I'll go with the separated configuration and I greatly appreciate that observation.
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An alternative approach to setting the volume level on the Rogue is to use the test tone from the av receiver and measure the output from each speaker with a Radio Shack dB meter. Set the adjustments on the receiver so each speakers is the same level. If possible, set the Rogue to a reproducible point and use the receiver adjustments as a secondary adjustment. Then, as Bob says, mark the spot on the Rogue and always set it to that point when playing surround. Once set up, you only need to use the receiver volume control. It may sound complicated, but it pretty straightforward after a little practice.
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Great instructions and i do have a sound level meter. I'm so glad I decided to put this question to Audiogon and I'll use this information to pursue my original idea which will be to combine the systems so I can keep the full range speakers in the system without having to add even more switches.

Presently I have the Martin Logan Odysseys as my stereo, or front, speakers with a pair of Jamo Concert 8 bookshelves which are selectable between the two using an Adcom speaker switching unit. This works out very well for me because I much prefer the bookshelves for jazz guitar and most other light jazz music. I use the full range speakers for most everything else. Thanks for your help; I'll post the outcome of this experiment.