Either the M2 or M3 would be great. People are very high on the M2, which offers excellent midrange and treble performance. The M3 offers more bass, which only matters if you listen to music without a sub.
There are two ways to decide. If you can swing it on your credit card, order both and just keep the one you prefer...shipping a pair of bookshelf speakers isn't going to run you big bucks.
If you can't be bothered doing that, let your musical tastes guide you. If you prefer popular music, the M3 will be easier to listen over a few hours. If you're a jazz or classical fan, the midrange performance of the M2 will probably make it the better buy.
Quadpole surrounds are amazing. You'll love the QS4s.
