I am away from home this week so can’t listen to my devices but I have a V4 with external synth (VZ-1) connected to input 1 without hum or buzz. I am sure I have configured the input as both TS & TRS. I tend to use the T+R setting which allows for mono TS connection but also stereo-to-mono of a TRS stereo cable. Both of these have worked well for me.
Try connecting each of your synths, one at a time to input 1 with nothing connected to input 2. You may find that they behave differently which would give us another clue.
[Edit] Also look at the alsa settings for the soundcard in webconf. Not all controls are exposed to the UI by default and there may be a misconfiguration of a hidden control. You can adjust each control from webconf and you can expose and hide each from the UI.
This is how my V5 is configured:
but I think the V4 (that I cannot access its webconf today) has VINL2[SE]+VINL1[SE] which I think means:
- VINL2: line input 2 (ring)
- SE: single ended
- VINL1[SE]: line input 1 (tip) single ended
It has been a while since I played with this so my memory of the actual settings may be off, but basically I want the tip and ring to sum for each input so that a TS mono cable will give mono (tip + nothing) and a stereo TRS (stereo TRS to 2x mono TS) cable witll sum the stereo source (tip + ring) to a mono input.
[Edit] Check out this post: Audio levels for dummies - #5 by riban