Any problems with the Mini Rectifier?

[WMHaze]

My Mini Rectifier has no Bass Control in the Vintage/Modern Channel anymore.
It doesn't matter if the bass knob is 7:00 or 5:00, it sounds the same.
It sounds as if it was a little before middle position, and sounds with a weak bass...
The clean Channel remains perfect.
Has anyone already heard about this and guess what may be?
Tubes, or something else in the circuitry?
Thanks

My uneducated guess is maybe the bass pot has gone bad? I don't know.