My understanding is quite general. But I think both Dolby PLII and Harman Kardon Logic 7 work best with acoustically recorded sound, such as a classical or live jazz performance, where each stereo channel contains some "information" about the other because each main steeo microphone picks up delayed or echoed sound that the other main microphone records more directly. The two channels are then processed to isolate these "live" artifacts from the more directly recorded music, then the whole thing is remixed on the fly into multiple surround channels as appropriate.
Much popular music -- I'll bet your Elton John recording is like this -- is created as much as in the engeineer's booth as in the live studio. In other words, which sounds are on which stereo channel is very much a function of what the recording engineer considered good stereo, and how he mixed multiple microphone channels into two on the final recording. That may bear little relation to what the live acoustic mix was like inside the studio.
So unlike a concert-like performance -- which likely is recreated more accurately with PLII or Logic 7 -- the artificially pop-mixed stereo may not play any better and may even sound worse.
