I saw you're posts in joker's thread and the EPH-100 thread, and figured you'd have a separate thread open. I recently had the EPH-100, RE-400 and GR07 MKII at the same time. A little background - my first 'good' IEM was the RE-ZERO a while back, which I loved but always found myself wanting a bit more low end. I then moved to the GR07 BE, which is very satisfying in the bass while still retaining a fair bit of linearity, but I found the treble to be a bit too offensive, aggressive and sibilant at times. I then decided to try out the RE-400, and after a few weeks with them and the BE, I found myself reaching for the RE-400 more so I parted with the BE. A few months later, I decided to acquire a GR07 MKII, then a month after that an EPH-100 to do some more comparisons. I have since parted with the all but the RE-400 as it fits my preferences best, but more on that later...I'll throw in some comparison of the GR07 but focus on the two in question the most.
The EPH-100 between the three has the greatest emphasis towards bass as you may have heard, but it isn't excessive in that it adversely interferes with the lower mids or muddies up the sound in any way. However, the high mids/low treble seems to be least prominent/laid-back overall of the three and so the elevation in bass can make this more apparent depending on the style of music you're listening to. This is not necessarily bad - as a result they're quite smooth and never offensive in that frequency region. I think the highlights with this IEM are in it's musicality, bass impact, warmth and the bass/mid transition. On the bass, there's plenty of sub-bass presence and it has a good bit of decay. I think in this particular presentation that's good because it makes the bass feel more real, more tactile albeit some may argue it 'sounds slower' due to the lengthier decay. I never really got that feeling though. There is a bit of a mid-bass emphasis but not too much...so you get a sense of impact but without much congestion.
This IEM is definitely one I would call engaging, musical and fun. Because of the bass body, the overall sound and mids are warm/full...never felt thin, basically. The texture and quality of the bass are better than most, but was ultimately bested by the more monitor quality of the GR07s and in a few instances the RE-400...still, I never felt they were lacking and for things like EDM, or any electronica I still preferred the EPH-100's more engaging presentation and ability to provide more tactile bass. The bass remains clean and controlled regardless of what you're throwing at these. The bass and low-mids align quite nicely and transition effortlessly even if they are pushed back a bit as a result of the bass emphasis...they never really sound recessed. The upper mids however are noticeably laid-back and while this lends to their smoothness it can have an affect on micro-detail and resolution here depending on the style of music. For example, these were my least favorite for vocals, but my most favorite for instrumental EDM. Still on their own, they're quite good as an all-arounder if you're after a more musical presentation.
Treble is relatively prominent compared to the rest of the FR but in a laid-back fashion (again, smoothness is the name of the game here) though they still manage to be pretty airy sounding. Not quite as much as the RE-400, and further from the GR07 but not by much overall. It's a small compromise, and a more favorable treble presentation from my experience with the GR07...but like the upper mids you'll loose out on a bit of resolution with them being more laid-back. The soundstage is quite nice...the GR07 was the wider of the three but had the least depth/height which I felt made it's presentation a bit odd at times. The EPH-100 is a bit better at balancing the soundstage and perceived distances/imaging capabilities (not as wide/distant sounding at the GR07) but not quite as good in this balance it as the RE-400, which seems to have a bit more perceived space/openness. The EPH-100 is well above average though, don't get me wrong...it's right below the RE-400 in most of these aspects. Both have a fairly 'spherical' and 3D soundstage, even if they're not the largest out there both are quite natural.
The RE-400 is the more linear, mid-focused of the three. The bass isn't flat, it's still a tad bit emphasized but has the least amount of presence of the three...if a track calls for bass, it can deliver (as much as a micro-driver can). Bass extension/depth is very good like the others but it's the least punchy of the three...the other two have more mid-bass comparatively. Overall, the mid-range is the most forward of the three. Because of the more linear bass, slightly forward mids and slightly less peaky treble (compared to the GR07 anyway), these are overall the least fatiguing and most relaxed of the three. Between the two in question, the sub-bass will be the biggest initial difference as the RE-400's is less forward and decays quicker...so it may seem less exciting as a result. In some cases, the bass resolution and texture I felt was better on the RE-400 but due to it being less prominent, these details can be missed in a complex song or when you've got more going on in the mids/treble. Bass/Mid/High transition seems effortless and never wonky/unnatural. The RE-400 does have warmth/body to it and just so happened to be at the right level for me...but it's noticeably less than that of the EPH-100.
As for the mids, they're the star of the show here. They're almost forward but not quite so, lending to a more intimate presentation in regards to vocals and acoustic instruments...but not overly so. The soundstage to me seems like a natural size whereas the GR07 was perceived to be a bit oddly shaped with more width than depth. Depth is well rendered here, I'd say better than the other two, which tends to be good for mids as that is where alot of instrumentation and complexity resides. As I mentioned before though, on overly complex passages in songs this may detract from other aspects like what is going on in the bass. The mids, I feel, are the most life-like of the three, and I think this is aided by the fact that they have a very transparent presentation and excellent imaging capabilities. Because the mid-range is more forward comparatively, things like vocals seem closer to you on average, or more like you're being presented with them in a smaller venue.
The treble presentation was for me the best compromise between the other two, sounding the most 'correct'and natural. The treble here is well extended, I'd say better than the EPH-100 and is a bit more resolving overall...it felt the most natural to my ear of the three. Well extended, never overly harsh/sibilant like the GR07 but not overly relaxed like the EPH-100 for my tastes. I felt the treble resolution was better than the EPH-100 overall, and a bit more sparkly next to the EPH-100 (in a favorable way) while still relatively smooth and forgiving of even my more 'intense' and poorly recorded tracks where the GR07 would almost be overbearing at times. I will say the EPH-100 has the smoothest treble, but I have yet to be offended in any way with the RE-400's presentation there. I would not classify these as bright IEMs, but side by side with the EPH-100 they're noticeably brighter. These just so happen to have that perfect compromise for me, especially good because some of the stuff I listen to doesn't have the best mastering or recording quality. They really do shine when you present them with some well recorded/mastered tracks, and really the same can be said with well matched tracks on the EPH-100.
Note: Most of my comparison has been done after I performed a modification to the RE-400 that involves placing a small felt disc in the nozzle, between the damping foam already there and the HiFiMan protective filter. This essentially attenuates everything above 1kHz by about 1-3dB (depending on the frequency) which ultimately bring everything below 1kHz a tad bit more forward. This adds a bit to lower mids/bass and a bit more warmth, without any perceivable adverse affects otherwise. PM me for more details.
Basically, this is more of a genre dependent thing, but overall I found the RE-400 to be the best compromise for use with multiple genres (and I listen to alot). Also, I think I'm more of a 'mid-head' than anything...which wasn't always the case, as I'd classify myself closer to a 'bass-head' at one point. HiFiMan has sort of a house sound going with their IEMs, which I've liked so far (I've heard the RE0, RE-ZERO, RE-262, RE-600 so far). I've seen joker mention a few times that both of these IEMs will cover all your bases, and I think that's certainly true given their strengths and weaknesses with any given genre. Had I not needed to slim down my collection and if I were using IEMs more, I'd have kept the EPH-100 along side the RE-400 because I think they work well together. Ultimately, I couldn't part with the RE-400.
Beyond sound, both have similar construction/durability, with the RE-400 being a bit smaller with a more narrow nozzle. The EPH-100 may have the upper hand in cable durability, but I'd rate them similarly. Fit will be determined by your ear canal, but I was able to get a good seal with both and I typically wear medium sized tips with most IEMs. Isolation was similar (and tip dependent) because both are ported dynamic micro-drivers, but assuming you get a good seal they should isolate about the same. If you are outside with either and there is wind, expect some wind noise due to the port. Both work just fine over the ear, and that is optimal for reducing microphonics which will likely be present when worn cable down. The lower half of the RE-400 cable has a nylon sheath, which helps with microphonics too. Because of nozzle size, the RE-400 will fit more ear canals and have a wider variety of tip choices...including foam tips which I don't think will work on the EPH-100 (not tried though).
Now, I've not heard any Shure IEMs yet, but what I've read of the SE-535, what genres you like (the bulk of them anyway) and the type of sound you're after, the EPH-100 may be the one for you. I wish I could offer more comparison to your current IEMs, but I can say that vocals and acoustic instruments will likely be presented better in most people's opinion with the RE-400.