i really like the humboldt. with the little wilson tuntots it was a match in heaven.
beside sonics, it was pefect power, very juicy. when i switched to sabrina x, it was still nice but far from unlimited. with the heisenbergs, it sounds unlimited again. only big amps sound like big amps, question of torque.
your question is good, since they are buildt out of the same parts, so in some ways its steroids only, but the subjective impresission is quite dramatic. much more stable, the realism, etc etc.
the humboldt is like you can watch them perfectly, the heisenberg is an invitation to be with them.
the comment of my closest audiophile friend who joined my journey was: the humboldt is a toy. well, that was with the sabrina x which is known to be powerhungry. the difference with other speaker and smaller rooms might be much smaller.
overall, i cant describe how happy i am still after 2 years. have heard a lot of other top amps since and never ever even thought about to change.
that said, a friend from germany has a very similar system....heisenberg, wilson, nordost....and evaluated a lot of pre amps with the heisenbergs and stated the heisenberg would sound a 1000 times better with the nagra hd pre hv than the stern.
cut the story short, big shock, i should get one of thouse in about 10 days.
i feel a bit stupid as the sound is perfect right now and dont want anything else. i am scared of the nagra pre, lol. part of the deal is to sell the humboldt and stern, so i have no way back.
will do an update when i know more.