Got a chance to do a little back and forth with the ER-4S and JH13 this morning.
From memory, the JH13 isn't too far off the UERM. They both have peaks at 10k but the JH13 peak is a good bit smaller. The other peak the JH13 has is at 6k and I believe the UERM was closer to 5k. The other main difference is in bass. I'd say the bass curve of the JH13 would follow a similar look to the UERM but add 2, possibly 3, db's evenly everywhere. Rin's measurements of the UERM show a small dip around 3k that I do not hear in the JH13. The overall effect is that the UERM sounded much brighter to me and less warm.
Switching from the JH13 to the ER-4S takes a little while to adjust to both fit and drastic change in staging. The JH13 sounds much larger, more enveloping and a good bit thicker. The ER-4S bass quality is superb and bass detail is very easy to distinguish. It truly is designed to let you hear every nuance on the recording; however if you enjoy a more live instrument feel to your presentation, it just doesn't have the palpable kick of the JH13. The increased overall bass on the JH13 definitely gives you more of the live venue flavor and what many term a more natural presentation. While all the low end details are still there, the trade off is that they aren't laid bare like they are in the ER-4S and becomes more about feeling and emotion than technical prowess.
Treble sounds just a hair brighter, just a hair, in the JH13. I think this is most likely due to the 10k peak but it however remains very fatigue and sibilance free. In contrast the ER-4S treble seems a little softer in note and, to me, silky smooth; so the JH13 shows upper range details easier, which helps for better imaging.
Regarding midrange, I don't find the midrange laid back on the JH13, as it has very good presence from 1k through 3k but because the ER-4S doesn't have the bass presence of the JH13, it sounds mid centric immediately after switching between the two iems. After several minutes of brain adjustment, things start to even out on the ER-4S; however I do feel like they are just a bit tilted towards the upper mid.
As for instrument timbre, I like somethings better on each phone. I like treble shimmer on cymbals a bit better on the ER-4S but I greatly prefer distortion guitar on the JH13. And while I absolutely love the bass detail of the ER-4S, the JH13 gets my head bobbing and draws me into the driving beat of the drum kit. Both really are superb with vocal clarity and detail; the freqphase thing really seems to work well here.
The JH13 is by far the most comfortable custom I've had and is the most comfortable iem I own. The ER-4S fit is fairly comfortable in its own right but takes a minute or two after insertion to adjust. Once settled I don't have any issues using the gray trips; however I would love to find the ER-4S sound in a silicone ciem.