Do speakers within the same series generally use the same quality of components? Should the high-frequency and mid-range sound fairly similar with two speakers in the same series?
I would think so. My experience is with Axiom, where I was able to do a switched a/b comparison of the M60 and M80.
Even though the drivers used were the same, the configuration and crossover were a little different giving the M80 a little bit more detail and openness through the midrange and treble. The biggest difference between the two was in the bass, with the midrange and upper end being slightly different. Looking back, if I had not been able to compare them directly, may not have noticed the difference.
I also had a chance to compare the M22 bookshelf and a sub to both the M80 and the M60 (both without sub). The bottom end on the bookself + sub was better in both cases. For the Flagship M80 is was only noticeable on music with content below 40Hz (things like synth and pipe organ).
If you are trying to decide between the Studio 100 and 60, but will be getting a sub, I would get a dealer to set up a comparison between the two setups to see if you can notice a difference.