I use a SLP-05 in this type of configuration. Connect you main speakers to the Cary/Adcom and the center, surrounds to the Arcam. If you want to use the sub just from home theater, just connect it to the Arcam. If you want to use it for both HT and stereo, it is more complicated. I would stick to using the sub just for HT to start.
First, connect the Pre Amp R and L RCA outputs on the Arcam to the Cinema In connections on the SLP-05. On the SLP-05, the right most toggle engages the Cinema In mode. It is simply a relay that completely bypasses the electronics on the SLP-05, connecting the cinema inputs to the outputs. You can even turn it off and the cinema bypass still works, just be careful if you are using the SLP-05 to trigger the Adcom. Personally, I just leave my SLP-05 on all the time. Then just redo the Arcam setup (crossover, speaker levels, filters).
You should connect your 2 channel inputs directly to the SLP-05. So for the Oppo, you connect the HDMI to the Arcam for BluRay and DVD and the L&R RCA outputs to the SLP-05 for stereo (through one of the normal SLP-05 AUX inputs). The DacMagic probably just goes to the SLP-05. If you want to do multichannel music (like DVD-A or SACD multichannel) from the Oppo, just use the HDMI input to the Arcam, like you would for a Blu-Ray. But, remember, using the Cinema In inputs, means that none of the SLP-05 pre-amp is used. The Arcam becomes the pre-amp for the L&R.
I find this setup works very well. The only issue is when I forget to reset the toggle when going from 2 channel back to the receiver, which means I have not R&L speakers.
Good luck. Nice system.