Believer,
I can't really explain what the sound differences are because the results vary so much. Basically, it sounds like the speakers get broken up into different sounding sections. But, like I said, the results vary so much, the sound is completely system dependant. The best technical and listening analysis I've seen was in the publication The Audio Perfectionist Journal. Unfortunately, its no longer in production, but the first few issues were free, and that's where you'll find the info you are looking for on vertical biamping. The journal was issued on pdf so locating it may be possible. I don't know what happened to my copies, or I would just give them to you.
That said, have a look at this: http://vandersteen.com//media/files/Manuals/3asigmanual.pdf
Its an owners manual for Vandersteen speakers. In it, he goes over the various biamping options and can do a much better job of explaining it than me. The info can be applied to just about any speaker, not just Vandersteen. Although, Pass does sound wonderful with Vandersteen.
