I haven't heard the EP175 so I can't comment on how it may sound against the EP500. You'll want to make sure phase and gain are set correctly at the crossover frequency for best results. Although my EP800 sounded very good "out of the box", it didn't peak in performance until I engineered its position and settings.
As for the center, in my system there is a world of difference between the 160 and 150 for both movies and music (PLII and Neo). I much prefer the simulated surround modes over stereo now that I have the 160.
