This is an update on my system. All your comments were very helpful.
This is a quote from a reply I posted in the speaker forum to someone who bought an AR HC6.
If anyone has any comments, they would be most appreciated.

"I purchased the AR HC6 speaker system too and am also impressed. It is a definite improvement, to my ears, over the Bose system I replaced.
I do have a concern with the sub's auto-off feature, which I wish was defeatable. Have you had your sub cut out repeatedly, like it was receiving signal and then not receiving signal? I tuned the system which seemed to help, but at times the sub will turn off with an audible 'thunk' (the auto-off feature in action. By the way, how long is the delay supposed to be?).
It appears the only way I can keep the sub on during an entire movie is to switch the crossover to 200, push the sub trim to nearly to 10, and raise the overall volume to around 45. Anything else and the sub turns off, and then sometimes on and off in rapid succession.
Do you think I should increase the sub signal in the calibration mode? Maybe this will get more signal to the sub and allow me to turn the crossover down to 150 or lower.
I hope it isn't a problem with the sub itself. That would definitely be disappointing given the excellent quality of the rest of the system.

IP: Logged"