Originally Posted by javier pintos

when yo connect only the channel b does sound come from both speakers (A and B), it could be that if the amp sees no input from the other channel it behaves like a mono amp (sort of like stereo channels on a mixer) and both outputs use the same input signal providing 40w+40w as output wich is not 80w.

I would guess that it only provides 40+40 and impedance should alway be 4 ohm minimum on any channel