At $1200, the Duo is 64 Audio's least expensive set of universal-fit in-ear monitors. I know this will get a laugh out of many, but note that the previous offering from the company covered here was the $3000 U18t from over 5 years ago that still sells for as much. I had a few different intentions for my testing of the 64 Audio Duo, including whether any of the tech from the U18t had trickled down and going with a dynamic driver addressed the shortcomings in the bass response of the all-balanced armature driver U18t. Of course, I was also curious about whether my dream of a DD + tia or open tube BA driver combination had come to fruition after all, which was another reason I was happy to examine the Duo when a review sample was offered.

The 64 Audio Duo mostly checks those boxes, although the dream remains lofty indeed! The replacement of the weaker woofer-style BA drivers with a dedicated DD for the low frequencies was a positive move in improving the bass response of the Duo compared to the U18t, so much so that this is one of the better examples of a hybrid set handling bass well for both quality and quantity. It does so by not even bothering with the analytic route. Instead, it is generally warm and smooth. The same driver fully handles the mids, as well as some of the treble response, which makes for a natural timbre and smooth transitions without an active crossover. The crossover is one of the more complex such executions too, which has the tia driver cater purely to the high frequencies for more energy in the presence region, as well as that sense of air, which works well with the DD placed closer to the ears than usual. Spaciousness is in fact a key element of the Duo, and Apex Core is an advancement over the fixed Apex module implementation in other 64 Audio offerings of the series, while taking key lessons from the adoption of replacement modules in the all-BA series, as with the U18t.

This makes the Duo a very interesting open-front, semi-open back set that leaves plenty of room for air to freely flow through the IEMs. It makes for some isolation when needed, and any sound can still get plenty loud, but for better or worse, you will always have an enhanced sense of your surroundings. As for the former, it allows for the Duo to be used comfortably for longer periods of time, even when going for a walk outside, but know that the bass and treble tuning will suffer if in a loud environment, which made it very difficult to tell which music genres best work with the Duo. That said, in a quiet environment, I found it to be extremely competent with the likes of rock and pop music, as much as general media consumption if working from home or otherwise. The technical performance is generally lacking for the money though, with several sets in the $600–1200 range often out-resolving the Duo. It's a very interesting set nonetheless, and there are innovative technologies, which have been detailed over the previous four pages. So whilst I am unable to recommend the 64 Audio Duo at its current asking price unless your use case perfectly fits its feature set, I am more than happy to award it our innovation award. The Duo is a mature set of IEMs deliberately aiming for something, and while out of the norm, it does get there most of the way.