I never noticed this before, but the output of my Select Series w/ 3.4 OS drops dramatically when changing patches. So much so that I must adjust the master volume with each new patch. As soon as I touch the master knob, even the tiniest bit, it jumps to the current volume.
It seems like it is getting sent a MIDI message telling the volume to drop with each patch change. But I unplugged the MIDI cables, power cycled, and the issue persists.
I have been playing with the VX-351 recently, which I am new to. But I disconnected all patches and power cycled again, to no avail.