Sound quality is without peer in my experience, but latest interface box is a step back.
I have been an audiophile for 48 years and this is, overall, the finest transducer (speaker or headphone) in my experience. I hear crystal-clear detail, top-to-bottom coherence, absence of resonances or tonal coloration, and masterful control in signal tracking through macro- and micro-dynamics. I would choose this in a second over the Stax 009S. I would also choose this over the Abyss 1266, mostly because it is so much lighter and easier to wear. Until someone figures out how to make a headphone version of a corona discharge/massless full-range driver that doesn't microwave cook your outer ears or poison you with ozone, I think this ribbon driver will remain the state-of-the-art.
As shipped from Drop, my unit came without any documentation or operating instructions in the Pelican case, but it is easy to figure out. Mine also had two changes from earlier production units. The first is a second, narrower leather strap, for the rear of the head. This helps secure the unit to the head, although the optimal length for mine is somewhere between the supplied hole/stud settings. Second, the amplifier interface box now has a second pair of binding posts as outputs for a pair of loudspeakers, along with an earspeaker/loudspeaker switch which is in series with your amplifier's output. This may be a good idea for those who have only one speaker amplifier and want it to share duties for driving the SR1a and speakers. However, it also potentially degrades the signal transmission, as my experiences with speaker-level switches have not been positive. It also increases the cost of manufacture. One more thing: when driven hard, the interface box "sings" with the music signal. I can't hear this while wearing the SR1a's, but if you put them in a ziplock bag and wrap them in a blanket to muffle their output, you can hear the box or its components resonating during certain signals. I don't know if there are inductive components inside (I've only seen banks of resistors in internal photos), but I am hearing vibrations due to inductive effects during large (>20 watt) signals. So as a bonus, you get a very inefficient speaker box with your SR1a's! I can't wait to try a Jotunheim R to bypass the interface box.
Would recommend to a friend.