I have done a few tests with the NT1000 on on a 4x12 halfstack. I was very happy the with results in that is sounded much clearer than a 57 and far more true to the room sound that other mics I have used in the past.
The best way I can describe it is that with 57's I always had to readjust the mic position and tweak the amp's controls to get the right sound. With the NT1000 I can make the amp sound the way I like in the room and then I've pretty much got that same sound to tape.