First of all, if you are using a tone generator and not listening to music, you are basically doing measurements and not listening, this is a different story. What you are saying is that you confirm the measured FR with another form of measurement, which is kind of redundant and trivial.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.
Eh yes, what he does is bringing it close to the target curve. I think this is clear, there is no mystery to itThis 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
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.
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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.
Strangely, in the link you provided the curves look a bit differently and very similar up to approx. 1kHz and all your talking mid-bass that leaks into lower mid end is not confirmed:
From this graph one can say that the only relevant difference in the FR is the additional peak at 5.5 kHz of the FD5 and a very slightly elevated region between 1 and 3 kHz. Is it audible ? In principle yes. But if you knew nothing about the FR of two unknown IEMs having a similar minor difference in FR as these two, listening to music and not test tones, I hardly doubt you would be able to pass a blind test. Again, I am with you, there is an audible difference (unlike in competent DAC and amps), but is is far from trivial to hear using music.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.
I recently did the Klippel test on distortion, with test tones and was able to get -50dB, with music there is no chance to come even close.