I found a used Definitive Technology PF1500TL for 375 and am just wondering if it would be worth it and how much of an upgrade over my Infinity ps-12 it would be?

No one has any opinions? How about a sub for under 500 that will beat my current ps-12?

I had a pf15 way back when :). Today, for $500 or less, one of your best choices is the SVS PB10-NSD. It is $429. The elemental design A3-300 might also be a good choice at $475. If you could possible stretch to just above $500 then the Epik Valor at $549 becomes an option and if you could really stretch then the AV123 MFW-15 at $599 would be an excellent choice. I would say more than any one of those choices would out perform the deftech. Like I said I had one and while it was good, I don't think it can hang with the newer options
EDIT: I am also confident that any above choices would beat out your current sub.

88 I currently have the PF15TL.

I have no knowledge of the infinity but the Definitive moves some air. If you current sub does not shake you in your seat the Definitive may do the trick. It was 6 or $700 new.