Before I went with the dual M6 sets I run now I had a set of M3 6.5" components originally in my front doors. When I got my first set of M6s they got moved to the back deck to replace the Kenwood coaxials I previously used then when I sourced the second set of M6s they came out of the car altogether.
I liked them just fine for what they were (the M6 comps just sound better, naturally) I think you'll like them fine as well. The aluminum tweeter does have a tendency to be somewhat on the brassy side depending on your source material but there is a setting on the crossover to attenuate the tweeter's output down a bit if you find it too harsh for your tastes.
I can't speak for any of the other brands you mentioned as I've not heard any...but personal experience with the Diamonds makes 'em an easy 'thumbs up' recommendation from me.
Be careful dealing with IndoAudio if you go with those Memphis comps. (not because of the Memphis gear, though) I have been hearing and reading about an inordinate amount of negative experiences that their customers have been dealing with here of late. Pay by PayPal and use a credit card when doing so, if you can, to double your oppotunity to get your $$$ back if they try and shaft you.
