"Please post some pics of what you are doing with the SRx ....:-)"
OK, this is a picture of what I was doing with the SRXIII pro.
On the upper left is the metal cover. It is made of thin aluminum and has the earpad glued to it. It is then held on the main body by two screws that also act as sleeves where the headband is attached. You can see one of these screws on the upper right.
So you pull the headband off, unscrew these two screws and pull off the aluminum cover. There are three scerws on the top of the main body( shown with the cord running from it.) which can be unscrewed allowing access to the driver. I didn't see much room in there for any sorb though.
When the metal cover is placed on the main body, there is a bit of space between it and the aluminum cover. Stax has some minuscule rubbery/sorb type dampers here. So I decided to try some 1 mm sorbethane there ( you can see one piece I cut out on the lower left and another on top of the main body.)
When I put it back together, it was a tight fit and hard to get the two screws in place so I extended the holes in the aluminum covers a bit by filing with a circular file. This took about 30 seconds of filing.
I put the phones back together, still keeping the previously added 1/8 in sorb attached to the outside of the aluminum cover. See previously posted picture showing a treated and untreated SRXIII.
The overall sound was too bassy and I went back and cut off about 1/2 of the amount of 1 mm sorb. This gave a much better sound now. Compared to the previous sound (with just the 1/8 in sorb on the outside of the aluminum) the sound now was smoother and less harsh and the bass seemed to be both more extended and sonorous. All in all a good sound and I spent the rest of the night just listening to these phones.
However I do want to try these phones without the outer ring of sorb, but with thicker sorb between the main body of the phone and the aluminum cover. The screw holes on the aluminum cover won't match but can be extended by the above mentioned circular file or just drilling new holes.
For the moment I am going on the belief that sorb is best placed closest to the drivers and here that means on top of the main body of the earpiece as opposed to the outside of the aluminum cover. I also have one extra SRXIII low bias with perforated drivers that I can use for spare parts.
Honestly folks, if you are interested in getting the best sound from your phones, this is a pretty interesting way to spend spend your time.This is not high level tinkering and as long as you take a modicum of care you shouldn't damage anything. All of the effects are reversible and sorbothane sheets are cheap (BTW it is about 1/3 the price on ebay and amazon than what the audio dealers are charging)
When I get a good result, which is about 80% of the time, I often just stop and spend hours/days or weeks just listening to the phones, enjoying the new sound or listening for subtle faults. It is super cool to be able to adjust favorite phones to your own preferences.
In general I would say, based on almost 2 years of playing with sorbothane, that it is most effective on cleaning up the sound of the middle and upper frequencies. The effects on bass are generally good but can sometimes go wonky. If so this can generally be corrected by reducing the amount of sorb.
However there are a range of other sonic benefits such as better space between instruments and dynamics as soren_brix notes.