I think (just my ignorant opinion) that a M60 on its side would be a mistake. The tweeter is a fair distance away from the woofer and I believe it would just sound funky. I’d vote for a pair of M3’s or 22’s. I’ve tried both and prefer the 22’s. The 3’s do have more bass, but this is an easy fix with the X-over. The 22’s blend extremely well with my 80’s which are supposed to be real close to the 60’s.

By going with a pair of bookshelves, you have to option to run them vertically under the screen, horizontally with the tweeter together, or the woofers together.

All that being said, I am very, very happy with my 22’s horizontally with the tweeters together. Looks like a big assed VP. But you will have to angle them to get them to point at your ears. I did this by ripping some MDF and building a shelf that holds them at the right angle. Took a whole thirty minute to bang it out.