I think the HSU would be a huge upgrade over the Axiom.
I have been told the lower axiom subs are not very good, no where near the 500 or 600, and I was even recommend by an Axiom employee to "shop around" when it came to the smaller subs.
A bigger sub (12", bigger box, etc.) will generally have lower bass, a bigger amp with generally have more slam (in a similar box)
I think the Hsu's are great values for the money.