I also owned a pair of the 906's and recently upgraded to the 907Be models. These speakers are excellent. That tweeter is just amazing. It is very smooth and yet at the same time very detailed. Something I have never heard before. Usually it's one or the other. You can not purchase the tweeters seperately and they wouldn't work with the old Electra or Utopia series even if you could. Although I think the Mezzo's are a wonderful speaker, if you can afford the new Be series, by all means do so. I wouldn't say there is a stark differnce between the two series, as there is a common thread. However, that new tweeter is NOT a subtle difference. The Be tweeter made me forget all about my old electrostats.
