Hey DjC, rack mount (and some floor stomp like Fx gadgets) offer up allot of possibilities. So lots of choices how and what Fx you may (or may not) want to use. They are very feature rich. One feature they share (and have for many years now) is they can be controlled by MIDI (Musical Instrument Digital Interface) albeit to varying degrees. I imagine you know something about MIDI ? Anyway the CC control stuff I posted about is part of the MIDI standard where some aspects of the unit can be controlled in real time (ie while your playing). Later ADA software versions offered added support for some of this and the MP2 came with it inbuilt (to some extent) from the get go (it was all evolving at much the same time in history).
Upshot is, that some parameters of various midi controllable Fx devices can be adjusted while you play (CC = Continuous Control) eg Master Volume (you can assign a midi compatible CC expression pedal to move it up/down while playing), other parameters you can switch on/off with a CC controller (button). So you can build a rack of toys, connect them to a midi controller (pedal) and not only change the patch they all go to at the press of "one" button but also change various parameters (on/off, louder/softer, wah etc). Now the MP-1s midi capability is a bit limited (as in it has no CC capabilities). ADA added some CC capability to the MB-1 (the bass preamp) in the later firmware (EPROM) updates. And the MP-2 has a bunch of stuff you can control this way.
So units like the TC GMaj2 (multi guitar Fx unit) also has CC capabilities, one of which is Master Volume control (but this controls its output, so if you chain through it (MP-1 > GMaj > Amp) it can control the volume/signal to the poweramp), handy, and a big part of why Mike went back to patching his GMaj2 this way.
The whole A/D conversion thing is (to some extent) a pedantic (but justified) aside. No matter how you convert analogue to digital (A/D) the digital is a sample/representation of the "true/real" signal. Of course this has to be converted back (D/A) to analogue to listen to it. TC (who make the MOne I use and the GMAj Mike uses) make good gadgets. However (and like many others in this space) the first thing the unit does is convert the "beautiful" analogue signal (coming from your MP-1) into digital so it can process it and then converts it back to analogue... Some of the older units eg Quadverb, Intelliflex etc maintained a direct undigitised (analogue pass through) route through the unit for the direct "analogue" signal. Does this sound better ? yes marginally, do you care ?, I do but up to you, however, it's good to understand what you are doing and why.