ER4XR

Super Review made a good video with comparisons and some "what even is neutral" stuff, comparing to various different neutral targets from different times. One of the timestamped sections is a comparison with the ER4XR: https://www.youtube.com/watch?v=rj6ZGT3fjA0 Crinacle also talked about the Etymotic Diffuse Field tuning in his own launch video. Bear in mind that the baseline DF that the SE series are tuned to has basically no bass whatsoever. It was based on a flawed understanding of what neutral was, and was really only accepted as neutral at the time because so many things were pushing the standard consumer-tuned bassy v-shape, so neutral was often seen as synonymous with "less bass". And less bass does tend to give better midrange clarity because of masking effects, but too little bass isn't neutral either. The XR series are actually closer to neutral. There's actually very little real evidence that JM-1 is perceptually neutral, so ultimately people are going to have a range of preferences (mainly due to HRTF differences) and some are going to find Etymotics do sound more neutral. Really beyond a certain point trying to chase perfectly neutral IEMs gets pointless because the only way to get true neutral is to start with something with fairly flat treble (so you're not dealing with a lot of peaks & dips) and then EQ it to your own HRTF. What the reference has going for it there is that it does seem to measure pretty flat up to around 16kHz and it seems to be fairly consistent between the 711 and 5128, which means it's not going to change drastically due to differences in acoustic impedance in different ear canals. However, from the looks of it, if you're going to EQ anyway, you can probably get similar performance from the Truthear Pure, which has very similar treble performance for a lot cheaper. I'm hoping to compare the Reference to the Volume S at some point, and I'll most likely go for the Volume S for its more fun bass presentation in the stock tuning and better comfort, but I can't say for sure until I've actually tried them both. Any one of those 3 would be a good candidate for EQ to flat neutral (with personalised HRTF).