I agree. I tested the BXD with my AK120 (optical out) and SE535 (unbalanced) at the shop and noticed the improvement especially with a more defined bass. Then I bought it and tried with my other headphones Senn Momentum, ATHAD900, the improvement is even more noticeable. Then I tested with AK120 line out to the BXD, the difference compared with using optical out is not significant.
In summary, I think BXD matches well with AK players and is a good upgrade for AK100 & AK120 players for better driving power and the balanced output. It's really not complaint for a portable DAC/head amp at this price.
I am using AK120 plus Traveller (at the first week, I was trying to get rid of the Traveller as I got another AMP but now I love it). As I like to use On-Ear phone, amp is really needed. AK120 is not able to carry the On-Ear phone (i.e. HD25 and DT1350). I recommend you get a AMP if using On-Ear or Over-Ear; however, if using IEM, maybe not necessary to do so.