Sub B & W 610: hummm problem

By attaching this sub to the secondary power outputs (spk B) of the amp I hear a great continuous "ummm", I think at 50 Hz, when the amp is switched off or when amp is on but with the spk B outputs turned off , to hear only spk A; hearing the music, with spk A and B togheter, all right; connecting sub with low-level input (signal) it's ok: is it normal, what can I do? Thank you!