Theremin does not work with new PA

I recently purchased a Kustom PA50 for use with my ~10 y.o. Moog Etherwave Theremin.  

The theremin line out works with my old guitar amp (doesn't sound great, but it works.)

The guitar works with the PA50.

My keyboard works with both the guitar amp and the PA50.

The theremin works with my stereo.

The theremin line out does not work (does not produce sound) when plugged into the PA50.

My local Guitar Center where I purchased the PA50 played with the setup for a bit but was unable to resolve the issue(s).

Any ideas?

Thank you!

The PA50 has 3 inputs. 2 of then have line and mic sockets
For your tests, are you using the same input for guitar, keyboard and theremin ?
Are you using the same cable ?

Sorry to just be getting back - touch of COVID.  

Thanks you and yes to both, André.  Using the 1/4" inputs and the same 1/4" cable.

It's physics-defying non-sensical.    The Kustom folks are mystified.  

Does the PA50 use a switching power supply ?
Usually, such a supply produces funny noises when used with or close to a theremin if the switching frequency is close to those of theremin's ocillators.
It could also mute it, may be.

Hmph!  I'll bounce that off the Kustom customer help guy (who is currently mystified.)

Thanks!

Are you using a mono 1/4" cable?  Stereo cables usually work, but sometimes not.

Can you get a 1/4" Y adapter and try connecting both amps at the same time?