Very interesting.
I tested this further now and found a perfect example, not with vocals but with flutes. In the Your Name soundtrack 三葉の通学 (Mitsuha no tsuugaku), this is painfully audible on all three headphones I currently test on.
For record the source is lossless 16 bit at a sample rate of 44100 Hz, so standard CD quality.
On the Atom amp, the flutes sound exactly as you’d expect. Super smooth and rich with barely any texture, if any to begin with. On the SP200 it almost sounds like a Cello playing with so much texture that it actually sounds like a completely different instrument that’s somehow broken.
Output impedance can be ruled out as both are very close to 1 ohm.
Has anyone got an idea? Testing those two amps this is literally the only thing that’s leaving me behind completely baffled.
@M0N with the incredible amount of gear you have heared and tested so far, do you maybe have any clue as to what this could cause? Not only because it’s just me but also @Alfredo3001 experiencing this with their THX amp.