Mid-bass waterfall or spectral decay as you call it is the first thing that I thought of in the conversation of high output impedance amplifiers and headphones with a high Q resonance peak in the mid-bass frequencies. I think of stored energy, resonance, smearing, color not just increased amplitude.
This may well be what you think of, but I hasten to reiterate my often-made point regarding the inherent tether between the time and frequency behavior of headphones - the stored energy yields an amplitude bump, the amplitude bump will be visible as longer decay time, but you fix one, you fix the other. If you like, I'll post some CSDs with minimum phase equalization tomorrow...
I have the impression that a calibrated microphone placed in the canal a fake neoprene ear may show us some of what is going on inside the ear cup of HD650 headphones. Neoprene may not make a perfect ear, I am okay with that.
In terms of analyzing the behavior of headphones, the ear is significant, though! What happens in the pad volume is dependent on the things in it, and the pinna is foremost among those things. If you want to look at what's happening at a point in space within the assembly, a probe microphone would probably be your best bet.
perhaps a sine burst as used by linkwitz when he tested drivers for his projects.
https://www.linkwitzlab.com/mid_dist.htm
If you want something equivalent and meaningfully more convenient, ARTA's Ivo Mateljan has a method to derive burst decay information from impulse response data
referenced in section 5 here and accessible to all ARTA users, including those with free/unpaid licenses.
This said, broadly, a focus on the time view of headphone behavior will ultimately run into the frustrating conclusion that we could have been looking closely at frequency response plots for this same information all along...
Yes, the parent company of AP purchased GRAS in 2018.
Jeez, and I like to think I pay attention! I may need to read a little more news and a little less AES.
Yeh the pinnae is likely different but the rest is identical to the 45CA down to where each screw is.
Makes sense to me - the build looked identical, but the pinnae do look different - and I couldn't imagine GRAS offering a Shore A-25 pinna on a headphone test fixture!