Long story short I have not been impressed with polk subs in my experience.
I've heard two Polk subs in my day the psw303 (which is more money than the PSW10). The other is an older Polk sub (12" driver @ 250 watts RMS, which was $350.00 new back in 2000).
I liked the velodyne VX-10 better than the psw303 in overall output, clarity, and extension, I ended up buying the VX-10. The 12" sub I've heard from polk is pretty boomy and does not have a very linear frequency response, I'm sure there's some room gain to blame for the sub performance but I think for overall sound quality my VX-10 is better than both subs from Polk.
It seems that Crutchfield and other dealers of Polk are always running "specials" where if you buy a Polk home theater speaker package, they will throw in a PSW10 for free. That has got to say something about that sub!
I have not heard the Onix x-sub.
Good luck