Here are my measurements and thoughts on the FX1800:
(DF-compensated) red = FX1800 stock, blue = FX1800 modded, green = FDX1 medium damped nozzle
Stock sound (red)
Let's start with the best thing: mids are gorgeous in the FX1800, sweeter and with even better vocal timbre than on the FDX1, while equally resolving. Certainly among the best mids I've heard.
Treble is also great, smooth and extended. With one minor exception around 5-6 kHz, where to my ears the emphasis is a tad overdone and makes some tracks sound borderline sibilant. I think it would benefit the sound signature to lower this range just a smidgen, in order to steer clear of sibilance, while retaining the same great treble resolution.
Bass is the range I have the most problem with. First of all, the hybrid DLC+wood driver appears to be a little "slower" than the FDX1 driver, so bass seems to linger a bit. Second, bass amplitude is about +5 dB higher than on the FDX1 throughout the entire range (from deep bass well into lower mids), which makes the low end sound noticeably boomy with some more bass-heavy tracks. IMHO, bass emphasis, plus somewhat long-ish decay, is definitely little too much for such a high-class earphone.
Therefore, I've been playing around with bass tuning quite a bit, and I think it should be lowered to FDX1 level. Or maybe just 1-2 dBs higher than FDX1 at max, but 5 dB higher is definitely too much. Note that even a modified FX1800 with the same bass level as the FDX1 will still sound slightly bassier than the latter, due to slightly slower decay.
Modified tuning (blue)
Flattening the peak around 5-6 kHz was pretty easy, I just affixed a thin strip of 3M micropore tape across the nozzle. In a production version, it might be possible to affix a damping strip like this directly to the protecting mesh, or just increase acoustic resistance of the mesh itself by a small amount.
Taming the bass was a little more challenging, but after some trial and error, I found a small piece of rubber band with just the right diameter to create the proper amount of leakage underneath the tips:
Mounting the tips together with the rubber strip is a bit tricky, but if done correctly, it should look like this:
Front:
Rear:
This tiny DIY front-vent underneath the tips creates just the right amount of airflow to lower the FX1800 bass to FDX1 level. Of course, in a production version, one wouldn't have to go to such modding lengths. One would simply lower acoustic resistance of the filters behind the existing front-vents, that's all (two holes in center of the photo).
Conclusion
I think, with slight adjustments to the tuning, these are some truely excellent IEMs. Not only do they look classier than the FDX1, but they also sound classier (i.e. not as technical and analytical, but overall sweeter and more euphonic, while being technically at least on the same level).