Patrcck, I was struggling with the same thought, too. I have Marantz SR18EX, and Sonus Faber Musica integrated. The easiest solution is;
1. Connect Receiver's "Pre-Out" to your pre amp's available input (Aux or anything)
2. Turn on and set you pre's volume where you can easily remember (i.e. 12 o'clock).
3. Turn on your Receiver and recalibrate the volume balance.
4. Whenever you want to enjoy movies, turn on receiver and pre/power amp, set your pre's input selector to that particular one at 1. above, turn the volume to the designated level (12 o'clock for example), and you can enjoy and control the volume just like you are/were using receiver only.
I hope this helps... BTW, you have a mighty great 2ch system comingle with home theater system!