I'll toss out my experience with these.

1) I don't have a subwoofer, but I run my mains full-range. I have not noticed a problem with bass info from the center surround, although to be honest I can't swear that I would notice.

2) I have never experienced a loss of sound when switching from 6.1 to two-channel (this applies to changes in input from the DVD player and to changing inputs from the DVD player to a source like CD or cable TV). I also have pretty much left 6.1 on for a year now -- any 5.1 DD source gets processed as 6.1 -- so I've had lots of opportunity.

3) The surround modes all sound different, and are all different from straight pro-logic.

This is using a Model 1050 purchased in early October 2000, if I remember correctly. I hope that helps some.