I agree with Metralla. Try moving the sub as well. Have you played with the crossover settings and volume on the sub? Try setting it lower, ie move from 80hz to 40. I would bet that your best results would be eliminating the sub, finding the best location for the speakers and then hooking the sub up to the second set of pre-outs if there are two sets of outputs. Then, slowly bring up the volume on the sub while running it full range. You won't have a gap at all and just have to avoid too much overlap.
A tube amp won't help your problem.