Jay,
My bookshelf M22s are about 7 years old. Besides the cabinet dimensions, they differ from the on-wall M22s in the tweeter and crossover. I really like the new tweeter. The top end of the on-wall sound is a definite improvement over my bookshelf M22s. The range where the bookshelf beats the on walls are in the lower midrange and bass, but the midrange differences depend on the material I'm listening to.
Like Ken mentioned elsewhere, the on-wall M22s image just as well as the bookshelves. I think they may even have better dispersion than the bookshelves -- again, owing to the newer tweeter. If I were comparing a new bookshelf to the on-wall, I'm pretty sure I'd pick the bookshelf. My wife, on the other hand, would pick the on-walls.
