That little 8" Velodyne SPL-II has a 1000-watt RMS digital amplifier in it, and moves serious air for a sub so small. I think the reason you're feeling the bass more is because the little SPL goes down to 28 Hz @ -3 dB, and Polk claims the PSW404 goes down to 32 Hz @ -3 dB.
It could also be that your friend's Velodyne might have been better placed in his room, compared to your own theatre. If you want to do an apples to apples comparison, see if your friend will allow you to borrow his sub for a few days, or take your Polk over and hook it up in exactly the same place as his SPL-II and see how it does. Room placement makes a HUGE difference with subs. Even my 12" SPL-II didn't quite satisfy me until I moved it about 6 feet.
Greg