No. Amps like the 875, and indeed most modern AV receivers, have preamp-level outputs for each channel. You simply run an interconnect from the front left/right preouts on the receiver to a spare line in on the Cyrus amp, and connect the front left/right speakers to the Cyrus as at present, and the centre, surrounds and subwoofer, along with your video sources, to the receiver. Leave your pure stereo sources - CD, tuner and so on - connected to the Cyrus.
Select the input on the Cyrus you've connected to the receiver's preouts, set the volume on the Cyrus to an easily repeatable position - I'd go for about 10 o'clock - and use the receiver's test-tones to balance up all the channels. The Onkyos, again like most modern receivers, will do this automatically using a test sequence and a microphone supplied in the box.
Then, when you want to play music in stereo, you use the Cyrus amp as usual, and the AV receiver is out of the system. When you want surround, you select the appropriate input on the Cyrus, set the volume to 10 o'clock, and away you go, controlling volume, etc, on the receiver.
Best of both worlds...