Just to provide some very, very brief overview of my experience of the RHA MA750, Yamaha EPH-100, and Sony MH1C. This is virtually all from memory so I can't make fantastic comparisons, but on averages they should roughly be right. Mainly used R&B and hip-hop tracks on an iPhone 5S, so for different genres recommendations may be different:
RHA MA750 - Only tried for a short while, so my pair weren't broken in. Once broken in they could change, but I think that the issues I had with them would still be present after break-in.
Very good bass extension. Goes very deep indeed. Not enough impact for my liking, it simply wasn't aggressive enough. Mids are very recessed, and lower treble is definitely accentuated. Overall quite a different sound signature to the other two, and not one that I like (reminds me of listening to an extremely compressed MP3). Enclosure was too big for me to get a long-lasting seal. Packaging was mediocre, but the build of the IEMs themselves was brilliant. Cable was lovely, accessories included were good, though the tips are definitely a little delicate and the case is a little rubbish (the zip is a single one that comes back round at the end, so you can actually zip it up unevenly). My advice for putting tips on, gently fold the bottom flanges back.
Sony MH1C
My "old" IEM. Bass quantity is a tad too much, often drowned out the mids. Good extension, and relatively good detail but quite slow. Difficult to say whether mids were a tad recessed or the bass took over, but very similar to the EPH-100 in this respect. More treble, and overall more balanced compared to a pair of EPH-100s with less than an hour on them (I'd expect this to change), but due to a lack of detail in comparison you almost don't get this impression. They sound rather congested, busy, and less open compared to the EPH-100s (again, with under an hour on them). Packaging is of course minimal, but at least it wasn't a pain. Accessories are basic, but again, OEM part. Flat cable is sort of tangle prone, but easily unravelled. The uneven weighting means they constantly fall out, and you can't fit them over the ear because the remote (which as you'd imagine doesn't work with iOS devices) gets in the way.
Yamaha EPH-100
Bloody awful packaging. Horrible sharp sealed plastic packaging to hold everything together (same as stuff you get from a hardware store) as opposed to a box (so once they're open, they're not going to be repackaged). The plastic tray to get the IEMs out of also requires a bit of a pull on the cable, unless you press through the back hard. Bass impact is great, and the bass in terms of quantity is a little less than the MH1C, but it's noticeably quicker. Definitely more agility and detail at the low-end. Mids appear to have more presence than the MH1Cs; it seems that there is better balance between the mids and low-end than the MH1C. Treble right now is more recessed (again, I would have thought this will change). However, as above, there is more detail, which actually gives the impression that there is more treble than there actually is (no bad thing). Cable is tangle prone, and rather thing - we'll see how it holds up, should be fine. Overall, sounds extremely similar to the MH1C, but has more detail, is more open, and is just easier to listen to. My only issue, as they're now discontinued, if they break, I have to hunt for another pair or look elsewhere!
If you really like the MH1C but want an overall upgrade, I'd strongly suggest skipping the MA750s and jumping to the EPH-100s.