Quote: Well, RE-400 are closest to RE-ZERO in tuning. What they did sacrifice for the (yet bit more than RE-ZERO) boosted bass is the highs extension of RE-272. However, they are less peaky than either RE-ZERO, RE0, RE-262 or RE-272. RE-272 indeed had a notable 4k peak and some midrange shout which could be annoying with some music. RE-ZERO had a smaller one at 3k. RE-400 is very linear (diffuse field) up to 5k if fitted deep. If you're interested, udauda has measured them and all the other ones mentioned too. Note that this coupler has exaggerated 3k boost, it's the same GRAS Etymotic uses and we know how ER4S are "accurate" yet have audible 3k boost. The author knows about it. Also 5-6k cut is personal - some people (like me) need a dip there, some don't. Whoever said GR07 or BA200 are superior to RE-400 hasn't listened to them. At all. I hope so, otherwise they're just plain wrong. It's not possible. GR07 aren't even more linear or extended plus they sound slightly "plasticky" - and the shell tends to prevent deep fit; BA200 are more linear but have some midrange boost like Etys, are a bit bit more midbassy; both have some distortion - in GR07, annoying 6k+12k ringing; in BA200, noticeable odd order harmonics in midrange. BA200 is tunable via insertion depth, but only for worse, while GR07 is almost not tunable because the shell prevents deeper insertion and only the peaks move around. You could mod BA200 (damper, impedance) to make them more neutral sounding, but they will still sound like a balanced armature IEM. And yes, distortion even below 1% is audible. 1% is -40 dB. You need to go less than 0.1% (-60 dB) in the midrange, preferably much less. The low distortion gives a "character" to the sound - either "sharpened" if it's high order, especially odd, like in balanced armatures; or over smoothed if it's low order, like in some cheap dynamic drivers. In bass, odd order can give the sound of fake speed, while even order will again smooth it out, but make it sound "stronger". The trick is, RE-400 drivers are highly tunable via the shell, much more so than RE-272. If you toss in major damping inside, you get a flat (as in non diffuse field) response that's yet more refined. Funny thing is that the shallow fit then brings them back to DF with less equalization. (with the large tips) And makes them actually airy, more than RE0 or RE272. The bass can be controlled by reducing venting making them more balanced, which is what I'll try as the next mod. RE-272 could be improved the same way, cutting the slight shout whiile trimming the 4k and smoothing out peaks. I think they should revisit RE-272 drivers and maybe shells (except not in the mediocre plastics and smaller) and add the new coating to linearize them, while slightly modifying the tuning to keep the highs and maybe bump bass. That could be RE-600. I hope it will be.