Interesting.
I measured the HD650's extensively (with many types of foam, felt, toilet paper, thick foam, K240 foam, stocking and the differences were very small usually below 2dB.
Also measured the HD58X with various materials and foam vs no-foam and double foam (rear foam on top of front foam)
Below
stock foam vs
double foam
View attachment 50051
and below
with front foam and
without front foam.(done with very careful repositioning, notice the
1dB scale)
View attachment 50052
It may be possible the foam has changed during production ?
The stuff that was on my proto and first production run looked exactly like the foam on the HD650.
Below my
HD650 experiments:
stretched nylon stocking, versus
standard foam vs
no foam, notice the 2dB scale. (Positioning wasn't done carefully)
View attachment 50053
Sennheiser foam vs
K240 foam (with cloth on the foam):
View attachment 50055
I have a few more experiments though.