Check that all the eqs are set not to cut bass, check there are no effects on and that the XPQ is off. If you still have the problem how sure are you there is no bass coming from the mixer? Have you tried it with another amp? We had one hire customer who had the same problem you describe and his amp had a crossover and it was set to act as a mid/top amp and wasn't giving him any bass, unsurprisingly, switching it back to full range sorted it.