According to the
owner's manual ...
The Bass frequency is a shelving type with approximately +/-12dB of effective cut/boost at 55 Hz.
The Treble frequency is a shelving type with approximately +8/-12dB of effective cut/boost at 4K Hz and approximately +/- 12 db at 12.5K Hz. In the boost position it offers the advantage of boosting the highs for clarity, without drastically increasing the upper mids, providing a more tonally balanced top end. In the cut mode it acts much like an instruments passive tone control.
The midrange band-pass ... filter features +/-12 dB of effective cut or boost with a bandwidth approximately 1 octave wide. The center frequency of this filter is user adjustable to 220Hz, 600Hz or 2.5kHz with the Mid Freq switch.
This seems to suggest that flat is 12 o'clock on all three controls.