Now that I've been listening to these for pretty much 24 hours straight, I think I have a decent grasp on how they sound. To date, I have not noticed any difference in sound signature since unboxing them.
Mids and treble are significantly more recessed than I was expecting, probably due to the bass overwhelming everything. EQ out some of the bass, and you will find that these are surprisingly detailed, however; you have to ditch the reason for buying them in the first place to really experience this, soooooo, take that as you will.
I have been using Gramatik's The Age of Reason primarily for my testing. The bass on every song sounds phenomenal as expected. The XB50 bully the FX3X all day in this aspect. Electric guitars prevalent throughout most of the album sound alright, but again they are overshadowed by the mountains of bass thundering around.
'Brave Men' is a standout song for these phones, especially around the 3:40 mark when the song transitions from electric guitar to sub-bass heavy electronic. I thought the transition was amazing on the 3X, but the XB50 shows them how its done.
Bluestep does a good job of exemplifying both the strengths and weaknesses of the XB50. There is a deep bassline that runs throughout the majority of the song, which sounds stunning. There is also a good mix of voicework and guitar, both of which sound quiet and much too subtle. This song better matches the 3X where the bass kicks hard, but does not overshadow the rest of the song.
So far I find the XB50 to be a very pleasant IEM, with tons of bass. They are extremely comfortable despite their absurd size, and not at all fatiguing to listen to. This is likely in part due to the mellow mids and highs. While I don't think they are worse than the 3X, I do think they compliment each other nicely.
Build quality seems spot on and the cable is a good length. The lined texture on the cable does produce some significant microphonics depending on what the cord rubs against. It is particularly noticeable during phone calls, per the other person I was speaking with. This is somewhat disappointing, but not the reason why I bought them. On the plus side, the cable feels exceptionally durable, and does not tangle too easily (IMO).
I must also note that they are exceptionally easy to drive. I actually use them on the lowest possible volume on my Moto G, most of the time. First IEM I've ever had that experience with. Great for battery life :D
For anyone who has these, please post your impressions! I am curious to see what you think.