Thanks Nigel. Which of your headphones do you like best with your M6? It seems like it takes about an hour for my M9 to sound its best after powering it up. I've been shutting the amp off when not in use. Do you do the same?
I love all my phones with the M6. One of it's strengths is it's flexibility to work with a lot of different phones (high and low impedance, power hungry or not) does not seem to make a lot of difference.
The natural character of the phones shines through and as I love them all it's hard to say which I like best.
From reading about the M9 it seems that it might be a little warmer but slightly less neutral than the M6 but I find the M6 to be superb, and not bright, with the HE6's, T1's and HD800's and at the same time no really 'dark' with the LCD2.2's although my cabling may effect that.
I normally warm mine up for at least 20 minutes before use but it sounds perfectly fine from cold. I do turn it off when not in use.