The most obvious question is whether you have calibrated the speaker and sub levels yet. The mains/center/surround probably won't be too far out but the sub level does need to be set correctly for sure.
If your receiver has auto EQ options TURN THEM OFF. Even the automatic level setting sometimes gets things wrong; the auto-EQ often just makes good speakers sound crappy.
Sub placement also makes quite a difference... you can gain or lose some huge holes in low frequency response depending on where the sub is placed. Have you done a "sub crawl" ?
Past that, I'm not sure what to say. If you were in a small room or had all the speakers arranged in a small part of your room (the room was 23x17 or something, wasn't it ?) and were playing movies quietly I can see how you might not notice much difference from the Encores, but in your large room at real "movie experience" volumes on a movie with a dynamic sound track (ie stuff blowing up) the difference would be night and day.
I'm assuming you have been watching some of the movies that can really show off a good sound system ? The English Patient ain't gonna sound any different on the 80/600 than it did on your Encores
