I write reviews only when I'm blown away with a product and this one is one of them. I tried the higher end model, T20i and I absolutely hated it. I thought I'd never buy RHA products again but I did. The 750i pair that I have got sound simply amazing.
 
The sound is a signature dynamic driver sound. It's not a very bassy headphone. Even if you consider it bassy, the bass does not bleed into mid or highs so the overall sonic signature is very clear. The treble is there and is not annoying at all (like the IE800s that I had). It's sparkly, fun and gives a sense of air to the sound.

They are very comfortable to use as they are over ear. I find the stock silicon tips to work the best. 
 
The cable suffers from some minor microphonics but it comes with a microphone!!! which makes it ideal to use with a smartphone. 
The best way to judge these is if you try them. They are available at Apple store so do it! You won't be disappointed 
 
 
P.S.
Sometimes, I feel confused in the world of audio! This IEM costs about $100 and it sounds better than a $300 JVC. I enjoy it more than my former IE800 which costs $700. I'm starting to slowly absorb the idea that the price does not necessarily correlate with performance. In addition, lower end models of a headphone company, in this case the 750s can sound better to some than the high end ones (T20i)!

I'll keep this short cause I'm not good at writing reviews. 
 
I just wanted to say that I have tried Shure 846 and I really did NOT like it. The sound from Shure is too condensed with not much soundstage (unlike its full sized cans which sound very nicely!) 
 
These IEMs (IE800s) are the ONLY IEMs that I've tried which sound like a full-sized headphone. You can't go wrong with the sound quality! 
 
Cheers!

Ok!
 
I'll keep it short. I think this player has:
 
1- a horrible button layout! Bad UI but that can be improved by installing a custom firmware.
2- Decent sound quality, not so wide soundstage! In fact, my laptop output (!) has a better soundstage than this DAP. 
 
 
Edit: I'm still analysing the sound quality. It might cause some reduction in the depth of soundstage.
After doing a lot of research and testing it on two devices, I ended up concluding that the soundstage is an issue on Fiio X3.
 
So, I returned it and got the Fiio X5. 
 
Fiio X5 does not sound that much different from my Nexus 5 ( ) but I'm glad it doesn't bassy as X3.
 
I also sold the Fiio X5. I don't think I'll go back to Fiio again.
 
The next stop is DX90 from iBasso.

Ok!
 
I saw that some people say that these headphones are unlistenable without equalizer and I completely disagree. I disagree to the extent that it made me write this review- something I normally don't do 
 
I'm no audio expert but I've found these headphones the clearest sounding among the ones I've so far tested. 
 
The special thing about these headphones it the fact that they have just "enough" bass. The perfect base allows the music to come to life; the mids and highs are extremely well detailed and clear because the bass does not distort them. A good competitor to Shure 1440  would be Sennheiser 598 which I find much less detailed because of the overpowering bass.
 
Maybe I just hate bass. Who knows?!