Thanks for the review. The best point of the E6 is that it seems to get rid of the low bass roll off that the E5 featured no matter the load. Thus to get a ruler flat frequency off balanced armatures the E6 seems to be the cheapest way (May be that's the reason for the Westone bundle).
If it just keeps up in distortion (Which apparently do) and it improves a bit the crosstalk performance of the E5 it might be a really strong bang for the buck gadget.
Yes. The bass rolls off is a side effect for the small coupling caps used in E5 / E7. FiiO managed to remove some of the caps because of the new power circuit so it runs more linearly now. That is one of the reason why it also sounds better than E5 / E7. As far as the RMAA test I ran between E5 and E6, the result are a general improvement on all front except for THD (0.024% > 0.032%). Thought number is so small it really doesn't matter. Most noticeable improvement are on intermodulation distortion + noise (0.299% > 0.073%) and stereo crosstalk (-78.7dB > -82.5dB). The result is of course a good explanation of why E6 sounds cleaner than E5.