Ok, so I just got a Mark III, and I see the reverb tank sitting in the bottom of the amp. Now so far, the reverb controls have not done anything, as in I dont' here any sonic difference. Is there something I'm supposed to do with the tank? I think it's functional, because when I turn reverb up, I can hear the springy sound if i shake the amp gently, and I dont' here this when I turn it to zero. But I just don't hear the reverby sound when I play.