First, Pro Logic II and IIx are essentially the same thing - whether you have one or the other is dependent solely on whether or not you have surround back speakers. I tested that by setting my 990 to the cable input, flipping to Discovery HD for a DD source (at the moment of the test it was a DD 2.0 source), and toggling the surround backs off - the active surround mode changed from PLIIx-Movie to PLII-Movie.

Second test, which I think is more along the lines of what you wanted to know... Let's say that you wanted to use PLIIx-Movie for watching TV (which is my standard approach) but wanted to use PLIIx-Music for CD's. I stayed with my cable input, changed to TNT-HD for a DD 5.1 this time (although by this time, after a break to re-assure Kate that it was OK to go to sleep, Discovery HD was also playing a DD 5.1 broadcast). In that case, my 990 applies DD+PLIIx-Movie. Then I set my front input to the optical 1 and DVD component inputs (so as not to monkey with my normal CD input, which is set to upsample) and set it to use PLIIx-Music with the CD that was playing at the time. I could toggle between inputs to my heart's content: Cable stayed DD+PLIIx-Movie (or PLIIx-Movie for PCM and DD 2.0 stations), and Video5 stayed PLIIx-Music.

Interesting discovery during this: Upsample works for more cases than I'd previously realized. Check this out: Take a Dolby Digital 5.1 source and toggle through "Stereo" modes until you get to Upsample (you'll run past 2-channel downmix and bypass, if memory serves) - you'll end up with the full Dolby Digital 5.1 active with Upsample applied. There's no way to select an additional surround mode (such as PLIIx) without turning Upsample off, but I'd previously assumed that Upsample was limited to PCM stereo. It's possible that flagged Dolby EX tracks would still kick in Dolby EX with upsample in the loop, but that's just a guess - I haven't tried it. Maybe I'll try Dolby EX, DTS, and DTS ES one of these days to see if they all work with Upsample as well...

Last comment of the night (I've got work to do and transplanted hostas to water): I'll summarize the way that I've ended up configuring both my 990 (and to a degree my 950 before it, although I used Cirrus Extra Surround instead of PLIIx and stereo instead of upsample in that case): DVD and HD cable are set up basically the same. Dolby Digital 5.1 sources get DD+PLIIx-Movie. Dolby Digital 2.0 and PCM stereo sources get PLIIx-Movie as well. DTS 5.1 sources get DTS+PLIIx-Movie. Dolby Digital EX and DTS ES Discrete sources both will typically force their processing modes, so those end up Dolby EX or DTS ES. CD sources are played back in upsampled stereo (although this evening I'm going to leave it on PLIIx-Music just for kicks). Tuner gets either stereo or 7-Stereo depending on my mood and the circumstances. Oh, and the iPod input (used mainly for background music during gatherings) gets 7-Stereo as well.