I switch to Mint recently from OpenSUSE. When I had the balance issue I popped open the Sound Control Panel and got what is shown below...no balance slider?? I figured it was somewhere else, so I looked a amixer, alsamixer, pusle_effects...nothing?...I had no reason to play with it and figured if I couldn't find it, it must be OK...The issue I was having only happened with the D50 in USB mode, TOSLINK worked fine, analog worked fine, a separate USB DAC worked fine?...I was stumped...
Digging around on the net I noticed a Mint screen shot showing the balance slider in the Sound Control Panel...huh?...It ends up that when the Panel is re-sized, it doesn't scale the contents, it crops....so the controls were there just out of view....dumb-ass....I still can't explain how the balance got changed....
I'd like to mention I had a email chain with Topping during this and they were awesome...great company!