Jack,
I'm not really familiar with any of the components you currently own, but I can tell you how to implement a system using a dedicated pre-amp integrated into your HT.
You would not want to use the tape loop of the Arcam.
First, acquire a pre-amp (preferably with an HT bypass/processor loop). You would connect your music source (naim CD player) to an input on the pre-amp, not to the HT processor. You would then connect the L&R output of the pre-amp into the front L&R inputs of your power amp. You would connect the front L&R output from your processor into the HT bypass/processor loop input of the pre-amp. You would connect any HT sources into the Arcam processor, not the new pre-amp.
OK, now for listening to 2-channel music, simply pop in a CD, turn on the pre-amp and amp and you are in business. You don't even need to turn on the Arcam processor for 2-channel music. You 2-channel rig would be totally independant of the HT system (but share 2-channels of amplification and the L&R front speakers)
Now, for HT or multi-channel, also turn on the HT processor and select HTbypass/processor loop on the pre-amp. This will route the L&R front outputs of the HT processor through the 2-channel portion of the system. When in bypass, the pre-amp simply passes the signal through to the power amp, so the HT processor affects the volume control of all channels of the system.
It is actually simpler than it may seem from my description. I've done it several times and it works well.
Enjoy,
TIC