Stemme had a beautiful voice indeed but, alas, a lyric one suitable for Sieglinde, Senta and the like. The shortgage of dramatic sopranos has driven her to heavy roles that have taken their toll on her voice. I think she would have been perfect for Leonore, but after much straining and pushing it's usual that a lyric voice shall not be able to sing its favourable roles. Soon she will be taking Brünnhilde and in few years we will see another gifted singer decline too early.
You cannot compare Stemme to Grob-Prandl, whot had a voice of pure steel - a real Hochdramatischersopran. Still, I feel she was too heroic. Historically speaking, Wagnerism has appropriated Leonore. This is role closer to Vitellia than to Isolde. I'll try to find out about G-P's failure to establish herself as a leading sopran.
Another "Fidelio" you have not mentioned is Erich Kleiber's live recording. A performance with much electricity about it. As for the Abbado set, criticism has been severe in Italy. The man has made "sound" his main obsession and a beautiful sound may detract from a Beethoven performance. In this, he has closely followed Karajan's steps. I do not care too much for "customized" voices like Kauffman's, but some days ago he made a fine impression in Valencia singing the role. I must admit he is really an interpreter. I alos liked him in the recent "Die Walküre" from the Met (at the cinema). I will try to listen to the whole thing.