Please write the techs at

info@ehx.com

Hi see if this helps:

The DUB LVL knob acts like a feedback control for the looper while overdubbing only. When this knob is at the maximum position, no volume loss will occur to the previously recorded portion of the loop. When the DUB LVL knob is set below maximum,each time the loop cycles during overdub the previous loop volume will be lower than the newly recorded overdub

From the manual page 25
https://www.ehx.com/assets/instructions/grand-canyon.pdf

Sounds like a dirty or corroded switch.
Deoxit spray cleaner can help .It is sprayed into the back and top of switch.

Please write the service techs at info@ehx.com

Here is one on Reverb. Maybe they will ship to your area.

https://reverb.com/item/30819180-electro-harmonix-hum-debugger-power-supply-ac-adaptor-eu-version-new-with-full-warranty

I will check with the factory on other ways on Monday.

Please contact the techsat
info@ehx.com

John the designer is no longer around.
So who knows!
You are right It should be high unless its designed for studio use.

This pedal is designed to be feed by a guitar not an effects loop.
Are you using it in an effects loop?