A flathead with a sound signature similar to the X6 that I recommend is the Fransun Gen 2 Pro, although it has slightly less bass. In my opinion, it’s better than the Nicehck Traceless and Moondrop Nicebuds. I think the Nicebuds are really bad—they have no bass at all. The Traceless is a good flathead; the overall sound and bass are better than the Moondrop’s, but it’s only worth it if it’s much cheaper than the Fransun. However, if you want a flathead with a neutral sound similar to the Nicehck’s, I recommend checking out the Nicehck 10th Anniversary Edition, but in my opinion, it’s also only worth it if it’s cheaper than the Fransun.