I can't really tell much difference, either, and the difference favors the 901! When I hooked them up side by side, flipping between vid1 and vid3 to compare, I thought PQ was cleaner on the 901, and the stereo separation was definitely more prominent and didn't have the big sound difference between mono and stereo, or between tv and dvd sound. Lately I've seen some color artifacts on the 901, smearing out high intensity reds. It's on a lower quality station, ION, which has 4 subchannels. I may have them on the 900 and just wasn't as sensitive to them yet. There are a few sound things I don't like about the 901, it seems like it lops off some sound, instantly dampening, it loses its accoustic vibration. There have been a few times when the volume unexplicably goes very soft and then builds back up in volume. It's probably part of the fix, but were it not for the chirp, I miss the fuller, more 'colorful' sound of the 900 now and then. Over the next couple weeks, with the 901 it seemed like I had fewer dropoffs in the late afternoon where one signal is strong but wavers. But a few more weeks later, the wavering is back, so I can't say for sure that the 901 is better with that wavering signal. Except for the initial side by side, it's all subjective, but I do favor the 901, all things considered.
ETA: I hear my sound through 3-5 speakers with a very old Sony pro logic dolby receiver. The main speakers are klipsch. The cecb is not hooked up to the receiver directly.