Blu-Rays (and everything else) sounds better in Multi-Stereo than on the Pro Logic II with my 5.1

Pro-Logic is not "true surround" iirc

IIRC Pro-Logic settings allow the receiver to take a stereo signal and interpolate it into 5.1

You're reciever should be set in some sort of Auto/Direct mode to allow the receiver to read the incomming input and output it correctly.

Pro logic would be used if you wanted to turn a stereo music track into surround, or a stereo show/movie that may not have a 5.1 track.

It's more normal than not for typical consumers. When choosing to listen to stereo sources in surround, the general public seems to prefer sending the same signal to all the speakers rather than sending extracted ambience to the surround speakers. The former is closer to what they expect "surround sound" to be than the latter. I've experienced this with friends and co-workers I've helped setting up their systems, seen it repeatedly in store demos, and heard it from people I know in the installation business. And it's usually for the same reason you mentioned: "sounds louder".

Multi-channel " stereo " modes sounding " louder " than discreet multi-channel modes is typical. What generally happens is that the SAME signal from a stereo source is pumped to ALL the speakers at the same level. The center is usually a combination of left and right and the back speakers will usually be mirrors of the left and right ( like the old quadraphonic. ) Of course HOW this is done depends upon the AVR brand. Surround modes don't sound as " loud " because the SUM TOTAL volume of all the speakers won't exceed the original signal volume by much. It is typically 70% for the Left, Center, Right, and remaining 30% for the surrounds. All this, of course, depends upon what is done in the mixing room. A lot of older stuff that was originally in Stereo and " up-mixed " to surround will actually sound " fuller " because there really wasn't any discreet surround data originally and it either has to be taken from the stereo mix or added in post-production, whether it is through additional foley work or digital sound effects.

I generally play everything in Audyssey DSX and switch to DTS Neo-X or PLIIz when something doesn't sound right. I rarely ever use the Multi-Channel Stereo mode now.

To me it is the opposite now, but I have PLENTY of power to go around. Playing something in 2-channel mode sounds flat and boring, even in 5/7/9-channel Stereo mode. Using Audyssey provides a little depth to the soundtrack/music but that may be because I'm not using any room treatments. These may be imperfections in my untreated room and would probably make the purists cringe. LOL

The only thing I'd add to this discussion is this: I'm a little confused as to wheteher the OP opts to downmix 5.1 content to stereo so he can hear it eaqually loud from all the speakers. If so, you lose sound effects placement. Something zooming across the back of the room ceases to be zooming across the back of the room. It's everywhere.

An alternative, especially if you don't have a receiver with a technology like Audyssey Dynamic EQ, is to simply turn your surrounds up a bit. In general, surrounds in movies are quieter than the front stage. What happens for those of us who listen at less than "reference" (ie freakin loud) levels is the surrounds become inaudible, or incompletely audible while the front stage is still fine. Adding a few dB to the surrounds will mitigate this reasonably and let you hear the mix the way it was mixed.