Tower speaker as a center channel?

I have two old Polk VM30 towers hooked up for music only (mostly classical and jazz) with a DSW600 sub and am considering replacing the smaller VM20 CC with another VM30 tower b/c recent changes to the room layout would make a tower fit better than a horizontal CC. I can't get bigger speakers b/c the wife won't let me. I don't have a TV hooked up in the room.
I don't listen to multi-channel music currently, so am using the surround sound processing of the Denon 3808ci to fake it. The music is streamed from Rhapsody through Sonos, but Denon does a great job at "decrompressing" it and it does sound pretty good to my ears. Will I realize any discernible benefit from adding a third tower for streamed music?
 
Thanks