I think the problem might be the Center Channel itself. Possibly a positioning issue. Have you tried it above and below the TV. Swap in one of your other speakers to see if the amp is driving the center the same as the rest.

I would like to think the VP150 would be an upgrade but I have not heard any polks recently. I did hear some when I was researching for my HT system 3-4yrs ago and found the Axioms to be just that much cleaner, more detailed than the Polks.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70