I have both the AK100 and the Mezzo (going back this week) and I must say that the Mezzo is better sounding in every single metric I can think of. Firstly, no noise from the headphone output. The AK100 has a low noise floor, but not a zero noise floor. The DX90's noise floor is slightly lower. Second, its internal amp really does have problems driving earphones. Audibly lower contrast is the biggest problem. The Mezzo mod brings it back, again with noise floor that is negligible. In fact, I would wager in all earphone use, it beats the DX90 for raw performance. There is, however, a small low pass filter, but that seems to be a function of the DAC. The original AK100 doesn't have this filter.
I will be sending my AK100 off for the optimisation. The difference is noticeable. And here in Japan, AK100 are selling so cheaply. People literally throwing them away for the new model. I just saw a unit in great shape go for 250$. iRiver ruined the value of their own players in one stroke. Prior to the announcement of the AK100ii, the AK100 was still selling used for over 500$.
It's crazy. So the benefits of the mezzo, if you can get a good price on a used AK100, or have one already, are huge, especially if you are a high-end earphone user. And, it has balanced output as well as single ended. Both appear to perform very very well.
Wow. A used ak100 plus mod seems like a bargain at that level of performance.