Alembic F-1X noise troubleshooting

[Lonnybass]

Hi gang-

I am having some trouble getting to the bottom of some noise that my F-1x is putting out via the effects RETURN jack.

I have typically run my rack tuner in front of the Alembic into the RETURN to take advantage of its mute function, but I am noticing a bit of distortion through my midrange when the signal is patched through it.

I've done some isolating of the rack with different wire-ups, different preamps and bypassing various elements of the system to try and pinpoint it, and the only time I can recreate the distortion is using the RETURN.

In addition to everything else, also tried running the tuner through the FRONT of the Alembic to see if I could replicate the sound - nope, sounds perfect, no issues.

I'm curious if anyone has any ideas what the issue might be here. Could it be an electrical component going bad? A tube issue? Something else?

NOTE: I have already opened up the preamp to hit the jack with contact cleaner and I have also tried the old trick of recycling the jack 30 or 40 times to try and remove any corrosion that may have built up.

Thoughts welcome - I would love to get to the bottom of this as soon as possible. Thanks!

Lonnybass