The FX3X has more in common with the old FX1X more than the FX101. If you polished up the sounds of the FX1X that is what the FX3X sound like. A much cleaner more refined sounding FX1X. The mids are clean n full on the FX3X and the bass just as brawny as the FX1X but again cleaner and more controlled in this area over them.. Unfortunately these also have the similar peakiness in the upper mids lower highs that are also on the FX1X..If they didn't bother you much on the FX1X these will certainly not be worse than what was on the FX1X.
Now are they worth the money? I would grab the Audio Technica ATH-CKM500 over these for about the same cost. While the CKM500s are not about big bass. They do offer solid bass impact and sub bass but has one of the best SQs for under $100 earphones to date. The differences in the highs and mids alone on the CKM500 are worth the cost of admission. Imagery and detail is on another level over over the FX3X. The FX3X is a good earphone and I am glad I got them but in all honesty for me at least they aren't a leap in sound over my CKS77.
These are more of a side grade than a clear upgrade. But if bass earphones are your cup o tea these are easily one of the top bass earphones I have heard to date and already beat up on my MMDTs and the MG7s. If these didn't have this peaky flaw in the sounds these would be tops imo but this flaw I am trying to beat out of them. We will see. To get a good idea of how these sound. Throw on your FX1X and hear them now imagine that entire sound cleaned up and you will get a good idea of how these sound.