Any midi foot controller will select between the modules and their channels.
The easiest way to do this would be to make sure all units are set to the same MIDI channel and connect them like this:
1. Controller MIDI OUT > #1 preamp unit/head MIDI IN
2. #1 Preamp unit/head MIDI OUT > #2 preamp/head MIDI IN
3. Repeat 2 and add as many units as you want
After that you just need to program (or map) the desired MIDI PROGRAM CHANGE command from the controller to a specific channel/module you want.
You can also have the ability to map MIDI CC COMMANDS to the channels/modules you want, granted that the MIDI controller has the ability to do CC commands, which almost all newer units do.
Audio connections is another thing, it will depend on how many preamps units/heads you have, if you want to do stereo, wet/dry/wet rig, etc.