Dual subs are better, but they have to be good subs! Polk is crap!!
Given how most people are on a limited budget, and also want to upgrade in the future, my advice for them is to buy the best single subwoofer they can afford (and waiting until they have enough for a decent one), and then they have a clear and easy and efficient upgrade path for bass in the future, by simply buying another one like it when they have the money and upgraditus. When one buys and sells off gear, one typically loses quite a bit of money, money that could have been spent on something that would cause one to have a better system in the long run.
So I say, unless you have plenty of money to blow right now (in the thousands), buy one really good subwoofer with all of the money you can spare for it (assuming, of course, that one already has good main speakers, which matter more than anything else). Then, you will have good and deep bass, and you will have an easy way to upgrade, if you wish to do so, without losing money.
