When I apply Dolby Pro Logic IIx to a 5.1 source to get rear channel sound (6.1), the effects coming from the right and left side surrounds diminish, and the effects in the rear channel are relatively stronger. Is this appropriate and to be expected? I did not this was supposed to affect the original channels in any way, or does it purposely soften the side surrounds to compensate for the rear effects?

See THIS (http://www.avsforum.com/avs-vb/showthread.php?t=958075) thread. Particularly my post at the very end. Have you calibrated your surround channels properly?

When I apply Dolby Pro Logic IIx to a 5.1 source to get rear channel sound (6.1), the effects coming from the right and left side surrounds diminish, and the effects in the rear channel are relatively stronger. Is this appropriate and to be expected? I did not this was supposed to affect the original channels in any way, or does it purposely soften the side surrounds to compensate for the rear effects?
Whenever the surround left and surround right play the same signal, it will
transfer into the surround back channel. If surround left and surround right
have different signal, it will play on the their own channels just like how left
and right speakers to center.

See THIS (http://www.avsforum.com/avs-vb/showthread.php?t=958075) thread. Particularly my post at the very end. Have you calibrated your surround channels properly?

Thank you.

Dolby describes the creation, per se, of a rear channel from the right and left surrounds, but does not go so far as to say the information is "stripped" from the surrounds. This explains what I hear, though. I recently moved my right and left surrounds from "high" to "low", and now I can put my ear to them and notice what is happening when I change my surround settings.

Whenever the surround left and surround right play the same signal, it will
transfer into the surround back channel. If surround left and surround right
have different signal, it will play on the their own channels just like how left
and right speakers to center.
That's not exactly how DPLIIx works. Read my post at the end of the thread I posted the link to.