I feel like you're describing the fx40s.
How are you driving them? What sort of music do you listen to?
I found the fx40s extremely sibilant, sharp and crisp in the highs, while the fx101s are much more tame in that area. I can't keep the 40s in my ears for more than 1-2 songs, while the fx101s are among the least fatiguing.
I've found that the 101s have the same amount of extension as the 3850, but are significantly less bloated and overbearing. Not that the 3580s are bad to begin with, but that the 101s come across more musically realistic.
That's very interesting. To me the FX40s definitely have a lot of sparkle in the highs, but the balanced sound and clarity make up for it. The highs on the FX40 cleared up for me with only about 20 hours of burn and are easy to listen to. I also think the FX40s have nearly as much sub-bass as the SHE3580s, but without any mid-bass hump and a lot less V shape.
I've tried driving the FX101s with just the output on my phone, the soundmagic A10, and the ZO2.3. All of them portray the same noisy "crashy" sibilance in the highs. One thing I did notice is that the hiss from the ZO is VERY pronounced with the FX101s and added to the sibilance.
All of my files are 320 or 192 MP3/WMA. Listening to everything from Minnesota, Pretty Lights, and Figure to Crosby Stills & Nash and The Nylons. Seal definitely isn't an issue. I'm getting a surprising amount of driver flex on insertion. The driver flex is along the same lines as the MMDTs.
Tough to say at this point if it's a bad pair or not. I'd err on the side of them being okay and it just not being an IEM that works for me. I'll just keep burning the bejeezus out of them.
I'd be interested to hear the Fx3x. I would think that the carbon nanotube driver in a bassy profile is something right up my alley.