The AX720 is more powerful than the Mixamp by a smidge, so yes. Not sure if the E6 will add any beneficial amounts of power as its a very weak amp. The E11 would've been a safer choice. The E5 and E6 I believe share the same power, and the E5 didn't even add an appreciable amount of volume to the Mixamp. People should realize that the difference between a few decibels in volume is quite a bit of power. Not sure how the math is, but I think in some instances, just a few decibels needs twice the power. The E5 barely made a difference when I added it to the Mixamp and trying to power harder to drive cans.
Ironically, I loved the K701 and E5 with bass boost on, though the sound was anything but K701-like.
For me the E6 added enough power to the mixamp pro to experience a noticeable difference. Without the E6 I feel like the mixamp is not really loud enough in some situations (depending on the source material I guess) but with the E6 I never run out of volume. So for me that combo works very well.