Thanks for the quick and informative reply. I am definitely looking to keep the front 3 timbre matched. The last few setups I've used haven't been which at one point would have been sacrilege for me, but surprisingly enough I've had pretty good experiences so far. The current setup is even matching the fabric dome tweeter of the CS2 with the cone tweeters of the rack-system towers up front
. As for subs, I've got JBL, Jamo and Klipsch 10" powered subs, A couple Sony 12" models, as well as a few passive 12"ers with external amps that I mess around with. The bass hasn't been a concern for me yet as even with the budget subs I'm using now I only turn them up about half way. I'm sure I'm missing out on a handful of low notes, but as of now the sub-upgrade is close to last on my list.
Another option I remembered is a set of Monitor Audio Silver 5i towers I have. I could probably find the matching center for $2-300. I also have a set of PSB 500i's that I was thinking of testing out for HT duty.
HTD, EMPtek, and Ascend have certainly been on my list. I really like the size of the EMPtek E56Ci Center. The HTD Level Three, Aperion Intimus 5C, and HSU HC-1 MK2 all seem pretty similar to my Polk CS2 specs-wise so I'm not sure they would be worth the money to upgrade. The Ascend CMT-340 seems like it would certainly be a step up, and I've read good things about that setup. I have also looked at the NHT Classic Two. Add into all those choices the DIY route, Madissound and Partsexpress both have bookshelf and MTM setups that would work good for L/C/R.
It's good to see what others have chosen in this price bracket, It seems there are hundreds of choices and unfortunately few reliable and in-depth direct comparisons between models.
EDIT: Just wanted to add that the setup is for 80% movies and 20% split between TV and music. Since most of the usage is for movies, I've been mainly concerned with finding a center that is a big upgrade, and then I would purchase the matching L/R