Hey MJMP,
The MPX series was released at the same time as the PCM 80/90, and it was the successor to the LPX series processors. In the 90's after Harman Group bought Lexicon, they went with dual processor engines in all the rackmount effects which used one processor for the reverb effects and the other for the modulation and time based effects.
The result was more transparent effects than in the earlier versions. The reverb effects in the MPX-1 are basically the same as the PCM-80, the only differences are the arrangements of some of the effects chains in the PCM-80. The MPX has a smaller DSP engine for the modulation/time-based effects, and what you have in the MPX is all you have to work with.
The PCM series at that time featured something new which was the addition of the memory card slot on the front panel. The PCM has a bigger DSP processor which gave the PCM-80 longer delay times, and the ability to split the delays into two separate channels in the effects chains, and the ability to parallel the modulation and time effects with the reverb effects, and send each to separate outputs from the PCM.
The memory cards were intended to offer upgrades to the PCM models instead of having to swap IC chips on the boards, as well as new effects groups for the PCM series, but I never saw more than two new memory cards for the PCM-80/90 back then before the models were upgraded to the 81/91 models, then discontinued.
Personally, I wanted the reverb effects from the Lexicons so I went with the MPX which was $1500 cheaper than the PCM 80 at the time. IIRC, when I auditioned both units in the shop, I liked the factory presets better in the MPX-1 than in the PCM-80 as far as the reverb effects were concerned. They sounded more like they were set up to just add to my effects chain and just go, whereas the PCM-80 factory presets sounded like they needed to be edited before they could be practical in the effects chain. One thing I always do when I'm shopping for a new effects processor, is to try and edit or modify a program there in the store, to see how easy it is to do, and I wasn't getting anywhere with the PCM-80. None of the sales people knew how to get into it either, and they brought out the manual which I thought was an encyclopedia book. So obviously, I went with the MPX.
Harley