Agree with using the receiver. I calibrate my sub into my system with sub level set to 0 in my system. I consider this my movie watching setting. Then if I need less bass I have a shortcut programmed into my Harmony remote to access the channel levels in the receiver and I back the sub off from there.
With that in mind, you could likely do the same with an RF Harmony remote that talks to an IR blaster behind your screen where you could turn your sub down directly. I would consider that plan B though as I am guessing you will not be able to easily return your sub to a known calibrated setting without being able to see where the gain is actually set. Hope that helps.

On-Wall M5HP LCR, QS8 & EP500 in 7.1