I've had a longtime interest (decades, really) in flathead earbuds. It is my favorite mode of personal listening because they are more comfortable for me than IEMs and headphones. Among those I've owned, my favorite is the Yinman 600. It has detailed but smooth treble and good bass (for an earbud). It is 600 ohm impedance with low sensitivity, and requires desktop amp levels of power for good performance. I actually need to turn up the volume control higher than I do for my HD6XX full size headphones! I have the original Yinman 600 from a few years ago, with MMCX connectors and reddish wood body. Discussions on Head-Fi say that the current version has a non-detachable cable and black body. However, vendors on AliExpress have continued to list it with pictures of the original MMCX version. You can ask vendors to send you pictures of the version they are actually shipping. Because of the non-detachable cable, you might also consider requesting a balanced connector (no extra charge) for flexibility, and use an adapter if necessary.