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@Wyville, I'm only able to compare the Phantom to the U12t. As he said, the Phantom is considerably warmer and fuller than the U12t. You'll hear tighter, more starkly defined instruments on the U12t, while the Phantom's emphasis on romanticism, body and tone will always come first. The U12t is quite a bit more neutral in tone. Its low-treble I believe is a
hair bright. In the midrange, the U12t is the inverse of the Phantom. It has a laid-back, neutrally-positioned lower-midrange and a raised upper-midrange. So, instruments will sound livelier and will project with more force. The Phantom's lower-mid emphasis gives it that gorgeously euphonic timbre, but instruments project more calmly by comparison; more chestiness and less mouthiness.
In terms of build quality, the A12t will come with a 3D-printed shell. From experience, it looks like the 3D-printed shell is quite a bit thicker. That's the only thing I can go on, lest I attempt a drop test to see which breaks first.
In terms of fit, it's worth noting that 64 Audio's fit is
much tighter than Empire's. If you prefer a more relaxed fit, I'd suggest you notify 64 Audio beforehand.