I think that some of it is personal preference. I've used both dipole (Paradigm Studio/ADP's) and bookshelf for surrounds, and find that I lean toward dipoles - I'm currently using dipoles for surround back and Axiom M3ti bookshelves for side surrounds due to mounting limitations in my space, but if there were no such limitations I'd still have dipole side surrounds and probably dipoles on the surround back.

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