sometimes when you are unmounting you have no choice but go down to bare rubber, the foam pad comes right off. I have sets that have some bare, some with foam because it stayed intact. (so sorry to my OCD friends here, I am sure reading that was like fingernails on chalkboard).
I have blocks that have tack-n-peel by tsuniko on them and I have plain blocks I use whichever I need for the stamp. I found using the t-n-p to be easier, less messy and less time consuming then trying to alter the rubber with aileens or putting a foam backing in each one. I personally found cutting that cling foam to the stamps was a mess and too much work - I have hundreds and hundres of stamp sets to unmount.
there are many threads on these forums talking about this, just search and you will get a wealth of info. Maybe too much