As the others have said.
If I were going to be able to buy a HSU or an SVS, I know I wouldn't be happy if I settled for anything less, just because there is always that "what if?".
Now I have never heard any Polk subs or the two afformentioned as well, but of all the HT forums in the known world that I read, I really can't ever recall any overly nice things about Polk subs, but tons about HSU/SVS.
And another thing, you would still have a $500 Polk for $200, that performed like a $200 sub, not at all like the $350-450 HSU/SVS.
Of course, I am not in your shoes, but I have been in similar instances several times, and I alway wish I had gotten the better one.
Colin