Keep the Polk and save up for something better. W/ a 6.5 active that velodyne even w/ the 8 inch passive and marginally more power is not going to be an improvement for home theater.
You need to find yourself a used SVS if you can pry one away from someone. Something with a 12 inch or better driver and decent power (over 100wpc continous - although many class d amp subs have a lot more). There's just no replacement for displacement when it comes to home theater subs.