It could be that the thump you're looking for are bass notes higher than what the subs produce. So, the problem may not be the subs, but the mains. The way I understand it, the goal of the subs is not to produce loud noises, but to fill in the missing lower end. A lot of the thump that I hear others talk about here on the Axiom site is the lower end of the M80/M60 speakers.

https://docs.google.com/spreadsheet/ccc?...amp;usp=sharing