One more vote for upgrading the center speaker first.

What frequency are you using now to cross over to the sub ? Might be worth bumping to 100 Hz to see if that makes any difference (and making sure the crossover on the Hsu is set to maximum).

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8