I see better QC in RE-600 (channel matching is perfect) and the added mass slightly reducing peaks and distortion at high levels due to increased damping. (silver coil)
The damping can be had in RE-400 by tossing some microfibre or doubling the filter - except this also increases acoustic impedance, while just adding mass doesn't - makes the IEM yet more dependent on insertion depth.
Fresh new "perfect" correction for very deep fit with the very short biflanges and RE-272 damper. I'm not sure about the exact magnitude at 20k, but it is close.
(EDIT: Updated, very minor resonant frequency and Q tweaking. Yet closer to ideal - but to be perfect I'd need a few more parametrics or that FFT graphic EQ.
Also the new crossfeed preset - have fun. The LV2 host I'm using is called Carla.)