Actually farfel is right that a) anything you're getting by running a stereo signal through a Pro Logic set-up is a potentially pleasant but unintentional effect and b) any Pro Logic receiver will also do this. Hell, I have an old quadraphonic set up and it does this too.

Now, many of you are asking how this works. Though I don't know specifically how the Pro Logic IIx works, I do know how Dolby Pro Logic works because it's based on a similar algorythm to one of the old quadraphonic systems (QS, for those keeping score.)

It's a bit complicated, but here goes...what a matrix surround system like Dolby Surround is based on is out of phase sounds. That means if you have two sound waves coming at you the waves are either hitting their peaks and valleys at the same points (in phase) or they are hitting them at different times (out of phase.)

When you encode something for Dolby Surround, there are some sounds that are intentionally out of phase. When you play them back through a stereo setup, you don't notice anything. When you decode them with Dolby, what happens is each of the surround speakers has it's own mathmatical formula for combining the left and right channels and pulling out differences between them which brings certain sounds out and pushes others back based on how they are phased.

One of the simplest surround setups (called a Hafler circuit) adds a third speaker which is hooked up to the positive terminals from both the left and right channel. What this does is just give the difference between the two channels and can sometimes bring out something in a recording that's burried otherwise.

OK, that's a VERY simplistic explination. Hopefully that makes sense, because that's the easiest way I can think of to explain it.

Now, as to why it works for a non-encoded recording, anytime you have a stereo recording, there are natural differences in phase between sounds in the two channels. A decoder, like Dolby Surround, will find these differences regardless of if they are there intentionally or naturally. Sometimes you get a good effect, sometimes you get little or something weird.

Anyway, enough tech talk for today. Sorry if I confused anyone more. Quadraphonic and multi-channel sound just happens to be a hoby of mine, so this is something I know a little about. It's just a little hard to explain how it works.