These are the changes made to the values of some capacitors and resistors, following the advices of some external people:
The value of these capacitors were drastically increased, to minimize the cut-off at 20Hz:
C2, C3, C5, C6, C8, C10
All of them are now polarized (electrolytic or film types?).
The values of these resistors, at the mixing point, were drastically increased, to not overload the op-amps:
R18 and R19
Signals at the outputs remain the same above 1KHz but are drastically improved in the range of 20 to 100Hz.
I'm still studying some advices about the biasing of the transistor.
But it's past time to go to bed here, in the Canaries...
Attached you have the new images and circuit files.
Note: To watch the images at full resolution, download them or left click first, then right-click and choose "View image".