I think acoustic filters should be superior in distortion vs electric means, at least low-pass filters. Distortion also gets rolled off by an acoustic filter while an electrical filter will just lower the signal, but not the harmonics. With HP filters it's most likely the other way around. Even distortion probably helps with cohesiveness. UERM with its highish D3 up to 500Hz or so (depending on how you crank it up) and then rapidly dropping distortion always sounds a bit more, well, distorted in the lower frequencies to me and cohesion suffers as a result. Well, the DD IEMs can have extremely low distortion, lower than open-back headphones - on the order of 0.001% at 100dB in the midrange. Possibly less. I wouldn't be surprised if a dynamic driver in-ear had less distortion than the Sennheiser HE1 (which likely isn't all that much better than an HD800 or SR009 in the midrange). I kind of doubt that for transducers more distortion can be better.