Tested on a 100w fender amp with black face circuitry (also tested outdoors):
efficiency, gain increase over the whole range, deep bass, crystal clear highs, more audible (but not annoying) mids due to the general increase in gain.
Little or no distortion up to the maximum volume (very light saturation, not unpleasant).
Very "technical" pro tubes but on the other hand the weak side is the timbre, less creamy and slightly harder.