I started this response almost two hours ago, but am just now ready to post it... Yeesh...

Dolby EX and DTS ES have always supported standard 5.1 tracks in the respective formats in addition to EX or ES-encoded tracks - EX really had to, since many early EX-flagged tracks were not identified as such (via flags or even labels on the packaging) due to concerns about confusing the consumer. Both EX and ES pre-date PLIIx, and can work well with 6.1 or 7.1 speaker setups. PLIIx, on the other hand, is a newer alternative that is solely intended as a support for other formats (stereo as well as converting 5.1 to 7.1). As part of that, Pro Logic IIx can be applied downstream of any processing mode, as long as that processing mode outputs 5.1 or less channels in the digital domain (which would cover PCM stereo from CD's or cable/satellite, Dolby 2.0 through 5.1 from DVD or cable/satellite, and DTS from DVD). Just to round out the weird world of PLIIx, it was introduced initially as an extension of PLII (carrying PLII from 5.1 to 7.1), but it can also act as an expanded replacement for PLII (so if you don't have a 6.1 or 7.1 speaker setup, you can still use PLIIx to extend from stereo to however many speakers you have available).