I concur with Bdgreg also. You already have everything you need to integrate a 2 channel system into your HT system.
I took a look at the back pannel of the Cary you have. Run two interconnects from the L/R main pre-outs on the Arcam AVR to the Cinema L/R inputs on the Cary. Then run another pair of intercommects from the Cary's main outputs into the Adcom amp. Then use the Arcams internal amps for your center, surrounds, etc. No swapping cables - the Adcom amp will always be used to drive your L/R main speakers whether you are listening to 2 channel via the Cary or HT via the Arcam. You may have to press a button. most likely on the Cary. Consult your owners manual.
I took a look at the back pannel of the Cary you have. Run two interconnects from the L/R main pre-outs on the Arcam AVR to the Cinema L/R inputs on the Cary. Then run another pair of intercommects from the Cary's main outputs into the Adcom amp. Then use the Arcams internal amps for your center, surrounds, etc. No swapping cables - the Adcom amp will always be used to drive your L/R main speakers whether you are listening to 2 channel via the Cary or HT via the Arcam. You may have to press a button. most likely on the Cary. Consult your owners manual.