Come on, you hear all that?? 'The FD5 is quite wobbly in the upper mids, it has peaks around 2.7, 5.4, 7.6kHz (for me)' So you hear the peaks at exactly 2.7, 5.4 and 7.6 kHz?
Yes, I can, using
a tone generator and going through the frequency range and listening for the volume going up and down. This is part of the process for generating an EQ curve, you want to verify measurements. Where the peaks are exactly, particularly the 8kHz peak (it's a bit lower, around 7.6kHz in my case) will vary depending on your individual ear anatomy and insertion depth- this is why I said "for me".
Try it yourself, I'd be surprised if you can't hear them. Do try it with the FD5 and tell me how many peaks you hear in the upper mids, and roughly where they are. I'd be very surprised if you come back and say the FD5 sounds totally level in that region and there are no peaks.
2.7, 5.4, 7.6kHz is roughly where they peak for me, I'm not claiming pinpoint accuracy here but they are around those points, give or take a few hundred Hz. It's very notable listening to that range that there are three peaks on the FD5 but only two on the Timeless, the first is also much smoother and it doesn't have a peak around 5.4. My experience with other IEMs is that excessive peakiness in that region tends to result in a harsh sound, and while the FD5 is not the worst for this, it is there. This is why Amir smoothed that out in his EQ with an adjustment UP at 3,700 and DOWN at 5,560, it brings up the first dip and down the second peak and smooths over that region. It's very effective and to me, makes the FD5 sound better.
If you look at the crinacle measurements they are VERY similar in FR, they basically differ only between 4 and 7 kHZ (and also above 10kHZ, but these measurements are never reliable).
Crinacle's measurements back up what I'm saying I hear, there is excess mid-bass that leaks into the lower mids and there is clearly a large peak around 5.5kHz that is not there on the Timeless. The FD5 has three significant peaks in the upper mids while the Timeless is much smoother, with a smoother initial ear gain peak and really only the coupler resonance peak. You can see this in the graph, the FD5 clearly has much bigger peaks in this region. I confirmed this by actually listening with a tone generator, but it's right there in the measurements you reference.
What isn't there on the graph is my subjective impression that the FD5 has more visceral bass impact, even when EQed down to Harman, that it has a more physical or tactile impression in the bass. I qualified that by saying it was my subjective impression and ascribing it to the driver type might be fanciful, but to me, there is a physicality to the FD5 bass that isn't quite the same on the Timeless, and that is the one thing for me it does better and that I go back to it for. This isn't saying that the Timeless doesn't have bass, it has plenty of bass, but there is a different quality to it on the FD5. And again- this is my subjective impression.
If you can not tell the FD5 and Timeless apart in a blind test, or you cannot hear any peaks in the FD5 response in the upper mids, I suspect you may have hearing damage. Not being able to tell apart a competent DAC or amp is expected- a good one is totally flat. There is nothing to tell apart. I certainly can't. But IEMs / headphones are quite different from each other. For me, the Timeless is better tuned stock, it is less bloated in the mid bass/lower mids and less peaky/harsh in the upper mids/treble. This is in the graph, and it's my subjective impression listening to it too.