My new system

My new system: Denon AVR 4520 CI Receiver
Polk RTIA7's bi amp
Polk CSIA6 bi wired
Polk RT600I surround (my old front speakers)

My question is the subwoofer. I have an older Velodyne CHT12 which I am currently using and it sounds ok, but is only rated at 170watts. An audio friend suggested I look into a Polk PSW 505 which is rated at 300 watts. There is a refurbished one on e-bay for $200.
Is the PSW as great as suggested? What other Subwoofers would work well with this system? any suggestions. Also would you go
refurbished with a subwoofer?