Hi Steve. I use the mono setting and have also been speaking to Des about sub problems. He asked that I send him the MSF settings for my 568.2mm which i duly did. I had set my sub to mono/mono/LFE (60/65/80)(large/large/large sides&rears) on the suggestion of my dealer and found the bass wholly lacking on DVDs. Restoring it to mono brought the bass back.

According to Des, the mono option delivers almost twice the db of bass that LFE offers -rather than "better quality bass." The bass was just inaudible with the gain level of my sub.
Given your experience, I am going to try the following : Revert back to LFE for 5.1 sub but significantly increase the gain on the sub itself. Then, in the settings, turn down the sub output for two channel sound, to ensure it doesn`t overwhelm.
Try messing about with the sizes of your speakers. I actually have a small centre and rears but find the sound is better with them set at "large."

G98, HD621, 568.25, 558.2, 559, B&W 803Ds, HTM2D, 4xSCM1, ASW850, Pioneer PDLX5090 50" Plasma, Cables by Atlas and Chord