From specs and picture, I believe SR1/2 share 100% the same drivers, pad.
(Except the very first day SR1 has only non-perforated pad version, then perforated pad has launched later for it).
..
The difference is what not relate to the sound: headband assy, earcup frame.
SR2 called "Open" while SR1 called Semi-open is the only one thing: how much sound leak -> it depeding on how much driver AND chamber were sealing.
I found (SR1) at "X" places, there are 4 layers of paper kind stick together->too much it block the air (it close, air can not pass) but lacking block the sound.
Semi-open but leak as fully open.
Dont know how many paper layer on SR1 driver, may be 4, may be 2 but not 5 (from the name "Open" compare to "Semi")
We can tune/moding by changing the number of paper layer at X mark places.
My SR1 at the 2nd day after purchased was punch air vent holes at the bigger paper sheet (the right X). There is reason for it:
I could NOT love the sonic perfomance of stock. Too much bass bleed and lack of high, can not produce piano low notes...too much "omph" comes into vocal.
When iBasso release perforated pad (follow users idea), I order then with this pad and air vents, it is OK for years.
I was thought that SR1 reach its max potention after mod (together with cable change material and 2.5 balance plug, pad rotate 45deg).
When digging more, I knew I was wrong!
This driver is sensitive with any thing that change pressure behind and infront of diaphragm and ear to diaphragm distance: pad thickness, air venting, acoustic room space.
I fully removed all paper layer at the left X, it will not resisit vibration of diaphragm any more. On the right X mark paper, open a triangle 5mm x 3mm -> a way to balance the pressure.
The pad is shallow (16mm), oval shape and not angled (flat).
I must forget all memories of how SR1 sound and relistening all album again! Night and day difference!