A bit late to the party, but the best mid-range I've heard from a pair of bookshelves is from Q Acoustic Model 300. Extremely silky and smooth. It's just too bad the speakers are lacking in other aspects like dynamics. The rolled-off highs and bass are also too big of a compromise for me.
I am perfectly happy with my current pair of speakers Cabasse Riga. I've upgraded from Triangle Comete 40th which is very well-known for their fantastic mid-range as well. I guess you do need to audition the speakers because each person might have different definition of what sounds good for mid-range. I prefer my mid-range to sound more natural rather than colored.
