Am currently in a bit of a quandary as far as getting 67 goes... I am getting nearly 4% aa per kill in mpg at the moment but have a friend bard who levelled to 67 and is getting between 2 and 2 1/2% a kill...
I only measure AA % when soloing so our situations differ slightly, but I am still suprised at the big difference your friend is seeing. Personally I have stuck at level 69 for the same reason you have stayed at 66. At 65 I was getting an average of 10% AA per kill in Vxed (or at the 65/66 Murkgliders in NC and WoS), Now that I have reached 69 this is dropping to just over 7% per kill. There is no arguing that staying at a lower level is far more productive for XP per kill, but the figures you have been given seem suprisingly harsh.
May I suggest that you ding 67 - see how you like it, and then take one or two 96% rezzes back to 66.99 if you find that things are as bad as you have been told.
For me, I'm happy sticking where I am - there are two or three 67 to 69 songs that I find very much worth the XP loss.
Hope this helps.
Xeona