Main Output knob not controlling left speaker (FT Duo)

[spicyhamster]

Hey all, new user here. I've had my Fast Track Duo for a couple years now, but have used the headphone jack exclusively for monitoring. I decided this weekend to finally invest in some decent speakers (KRK Rokit 5). Now that I've got them set up, I've discovered a problem: the Main Output knob isn't controlling the left speaker. It stays at a constant volume no matter how high or low I turn the knob. The right speaker changes with the knob's setting as you'd expect. How do I fix this?

I've tried:
Switching the L and R speakers
Different cables
Direct Monitoring ON/OFF

...which tells me the problem is not the speaker. Is there a setting on the Fast Track I've missed? Any help would be greatly appreciated.

Windows 10
Pro Tools Express (10.3.4)
Avid Fast Track Duo
KRK Rokit 5 (x2)

[spicyhamster]

I've pretty much come to the conclusion that my unit is faulty. Seeing as it's past warranty, I'll probably be buying something else in the future, or learning to live with the issue.