Howdy and welcome to AH! Can you explain how you will set this up? I was under the (perhaps misguided) impression that in order for something like this to work, the integrated amp would need something like a home theater bypass like on the Parasound Hint 6:
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So for HT, would you have a connection from a pre amp output on your AVR to an input on the Arcam/Yamaha, which would in turn be at a fixed volume from which the volume is adjusted via voltage output from the AVR?
BTW, the prevailing opinion here on AH is that an amp is an amp is an amp (i.e., no sound difference if operating within respective parameters) and that you would be wasting your money (I am not saying either way). That being said, I am very familiar with the A-S801 and think is is a fantastic piece of kit. I do not have first hand experience with the SA20, but am curious as I like Arcam's stuff...