If you check on the WM website, the RX-series is on the same page as the R-series. Whereas the R is a pretty standard recirc design (a la ER RC-series), the RX adds the bubble plate along with the intake silencer and a true union ball valve for the water feed. The water feed is also moved from the near the top of the skimmer body to the bottom underneath the bubble plate.
Looks like WM is going in whole hog with their skimmer line. They're even going to have a cone skimmer:
http://www.reefcentral.com/forums/sh...readid=1487970
Good to see a company not half-assing things.