6" is fine. I take it that you don't want to consider surgery on the shelf to create more vertical room.
Excellent article from the Audio Critic, by the way.
Nope, no surgery on the shelves. I have a five that match and cutting two up would not fly with the wife.
I like the Audio Critic webzine a lot. They cut through the quasi-religious audiophile nonsense and replace it with meaningful measurements and sound analysis. I also agree with their assertion that once you have a good source and an amp capable of delivering a clean signal, both of which can be had for reasonable money, the only thing that matters is speakers. It's the only part of the chain that makes a measurable difference to the sound.
Thanks for the feedback everyone. Looks like I'll be ordering up a set of M3's to audition. Bonus time is right around the corner and new speakers are going to be my gift to myself this year. Now I just need to decide what color to get.