That's how I did mine as well with regards to crossover. Trim to "flat". It doesn't really matter where you set the volume, in my experience, Audyssey tames it to -12 no matter what.

After the Audyssey does it's thing, adjust the sub volume to taste. Don't worry about damaging the driver by overdriving it, the DSP prevents that, a pretty reassuring feature!

"Never, never, never give up "... Winston Churchill