Del,
I have 2910 also and have been through the same setup quandries. First you need to set bass enhancer to ON to make the LFE channel active for the discrete analog out for the sub to work. I also found that I needed to engage the +10dB boost to get my sub channel into the correct ballpark for output (then fine tune with the speaker levels screen) if I didn't do this then I did not even get enough sub power to trigger the auto on circuit of my sub.
Hope this helps and I remembered everything correctly as I am not near my system at the moment. Congrats on a great system by the way.
Stuart
