I recently installed the TL-350 set in my store's demo room, and the set sounds fantastic paired with a DSWPRO440 sub. The 550 sub would provide a little more uumph on the lower end, but I'm trying to keep my demo accessible to those looking for a great cost effective setup. The highs are not quite as crystal clear as I'd hoped the ring radiator tweeters would provide, but it's still a good sounding set. If you cheat and turn up the highest frequencies in the receiver's eq, you can compensate pretty well.
I haven't heard the Def Techs, but I would imagine that if they're in the same price bracket, they're pretty comparable.
The Polks are very well made, but more of a hard plastic than aluminum or something. My favorite affordable satellite set is the KEF KHT3005, but it's about $1000 more at MSRP.
The Polks are pretty punchy little speakers by themselves, if you wanted to install a set for ambient music somewhere, but they certainly require a sub to sound good in a theater setup or for serious listening.
Good luck! And enjoy whatever you choose.