Sounds odd. Dolby PLllx shouldnít effect your bass output, AFAIK that is still being handled by your receiverís bass management system even when DPLllx is enabled. Sounds like you might have an outlaw 990 which does have the capability to attenuate the subwoofer signal by up to 8dB when in stereo mode for music. Perhaps thatís been enabled and for some reason when you switch to DPLllx itís cutting out and sending the full signal to the subwoofer.
Sorry if that isnít much help but what you describe shouldnít be happening.
I've been a religious 2-channel music listener but have recently been playing around with Dolby Prologic IIx Music instead. I have to admit, I think I prefer it now that I've dialed in the front soundstage.
How did you get the front soundstage dialed in. I just canít get it to sound right for some music. Works great for classical concerts and ambient music but with good studio recordings it makes the front soundstage seem flat or compressed. For a long time it made vocalist sound 5 feet wide until I figured out the default settings spread out the center information. Even though Iíve gotten the soundstage balanced out I just canít get past the tonal differences itís causing in the front soundstage.