I hate posting stuff that could be somewhat anti-Polk, but if there's one area where people here tend to prefer other brands it is for the Sub. I'm a polk-head through and through, but have bought polk more for the detail in the mid-upper end in the past and mostly for 2-channel setups and sound recording.
I've also never bought a real sub before (have a cheesy one that came with a 5.1 all-in-one thing). So honestly I'm a blank slate here.
I'm seeing similar prices on the PSW10 for Around $100 shipped and the same for what appears to be a clone of the Energy ESW-10 from a place called thespeakercompany.com. The ESW clone is here...
I've read good things about this one elsewhere and about the Energy that it is a clone of. I will say this, it has more power than the Polk Sub for the same $$, plus I can return it for no cost shipping if I don't like it. I am worried though that in the meantime the $99 Polk deals (w/out free return shipping) will dry up.
I'm really torn on this one and money is tight for me.