I own a MIDI equipped Quad.
The PCB is installed in place of the small reverb tank (which, of course, is missing). The MIDI connector are soldered directly to the PCB.
I don't know if retrofitting a non-MIDI Quad is all that interesting. The MIDI interface is quite old and it's not that flexible.
First off, you can just turn on channels (RHY1/LD1/RHY2/LD2) and EQ, you can't control REV cancel/boost, which may or may not be a problem depending on how you use this effect (I just leave it on with small mix settings, something between 2 or 3).
Secondly, you can't configure anything. You just have all preset and must stick with this. For example, MIDI PC 1 stands for "RHY1 EQ OFF", PC2 is for LEAD 1 EQ OFF", PC3 is "RHY1 EQ ON" and so on like this. You just can't configure it to have, for example, LEAD 2 EQ on on MIDI PC 3. This bores me quite alot because forces me to stick to a presettable MIDI controller, rather than a simpler (and smaller) controller like the Midibuddy.
Not to mention the lack of the small rev tank. I have no idea on how this effects the reverb sounds, but if you love your Quad's reverb I would think it twice before applying the mod.
I don't think you're still interested in these pics anymore, I just posted my message because it looks like no one here knows these MIDI equipped preamps and just wanted to leave this trace for the future, anyway if you need them, just let me know, I'll see what I can do.