Adding a third, center channel for dialogue to a 2 channel rig


What are the most important considerations one should be mindful of when purchasing a speaker to be used as a center, third speaker for movie and TV dialogue? My current system is 2 channel, using Ohm Walsh 100 S-3 speakers. Thank you in advance.
dante7
You will need a HT Processor or receiver to decode multi-channel source (such as bluray or satellite).
the center should  match as closely as possible your fronts in terms of timber--ideally i'd use an ohm walsh as the center (or another ohm model that closely mirrors the frequency response, dispersion, etc. of your fronts)