Don't get a Polk sub, they pretty much stink. The BIC's are quite a bit better. But for the money the Klipsch RW12D is the front runner right now.
Any sub will go with your towers. A sub is a sub. You don't need the same brand as your other speakers.
No real difference between front and down firing as far as sound goes. Down firing is better if you have curious little fingers that like to poke at speakers. Some people will try and tell you there's a difference in sound but the numbers don't support their conclusions.
Television: Mitsubishi WD65737 DLP
Processor: Emotiva UMC-200
Amps: Carver AV 806x/Behringer EP4000
Mains: DCM TimeFrame 600 Center: AT 453C
Surrounds: AT 251.1 Sub: Danley DTS-10
Blu Ray: Panasonic DMP-BD655