The tone control circuit used in the concert series is the same circuit that Sunn used in the 2000s. It's a modified "tone stack". Very basically...
The treble frequencies are split out through a separate path and the Treble control is wired in a "cut only" type fashion - opposite of the normal Fender/Marshall tone stack. The output signal is taken off the Treble control just line the more common tone stack.
The bass/middle frequencies go a separate path. The Middle control is first in the line and it is wired as a "treble cut" with a lower cutoff frequency than the Treble control. After that, the signal proceeds to the Bass control portion, and then to join the treble frequencies at the Treble control.
There is a small resistor in the Bass circuit that attempts to leave a little of the signal in when the Bass control is at zero, but for most purposes, the Middle control will have very little effect at a zero bass setting because very little signal is actually going through to the Treble control. Turning the Bass control up a bit will make the Middle control have more response.
I suggest starting out with a Bass setting of 2, a Treble setting of 10, and set the Middle control to whatever sounds good first. The Treble should be reduced to get a mellower tone if necessary. Then set the Bass control to whatever sounds good. At least this is the way I do it.
It doesn't sound like a glitch - just the way the circuit works.
As for your other heads... you didn't say what they were. The Beta series have an active middle control that has a separate path to the output and the Bass and Treble sides each have a separate path... so this type of circuit will be less interactive than the concert circuit or something with the Fender/Marshall style tone circuit (like a Model-T).
Hope this helps.
