ESX-8MD Clock Issue

[os]

But even if I found a way of doing what you suggest, the key point is that the moment I highlight ANY other track in Logic, the ES-3 / ESX-8M cuts out. Why should that be? Other software or hardware doesn't behave like that in Logic.

I imagine it's Logic shutting down the plug-in because it thinks it's inactive. It doesn't know that it can generate output without any MIDI input. For this reason, the effect version of the plug-in might be a better bet. Or maybe there's a way of keeping plug-ins alive in Logic - maybe using the I/O plug-in to output the signal instead of the track output.