Stereo Upmix Rear Channel Levels

I'm using Kodi for Windows.

I use the stereo upmix feature (system-->audio-->stereo upmix on) to get 5.1 audio from my stereo music.  It does a really good job of creating a nice surround effect for music that's not natively multichannel.

My issue is that the rear 2 channels are consistently about 7db too loud when using Kod's stereo upmix feature.  When I play native multichannel music or videos, the channel balance is correct -- the issue only occurs when using stereo upmix.

I can do an adjustment using option-->channel level adjust on my Denon receiver to reduce the rear 2 channels by 7db each.  However, the adjustment doesn't stick, so I need to do it every time I power on the receiver.   I don't want to do the level adjustment in the receiver's main speaker balance setting because that would make the rear speakers too soft for native multichannel sources.

Is there a way in Kodi that I can adjust the volume of the rear 2 channels when using stereo upmix to correct this in a sustainable way?

Thanks for any help or suggestions!