There was a post out a few years ago about the reduced shielding on the M3s-don't get closer than six inches from a CRT. Not aware if that same stipulation applies to all the other bookshelves, but it sure did for the M3s, had to run mine vertically and inverted to function correctly as a center-and it does that quite well as long as the room isn't too large.

But Yup, there's an ancient post out there talking about it.