Heard Robben Ford play his in an auditorium (in Hong Kong) with good acoustics and a very good sound engineer. Engineer didn't have much influence on the Dumble's sound as Ford had his Dumble cranked so loud it was all the engineer could do to bring everything else up to match it.
Larry Carlton wasn't brave/foolish enough to bring his Dumble to Hong Kong, IIRC.
Both players sounded good, like themselves.
So while the audience may not get a true representation of the Dumble sound in many cases (due to venue, engineer, FOH system, whatever), how the guitarist reacts to his instrument through a Dumble might be all that matters.
How many right here have heard little if any difference when a friend is A/B-ing two amps/pedals/instruments, only to FEEL the difference once it's their turn to play through the amps/pedals/instruments?
Is it a placebo effect? In some cases it may be.
I've tried some things and it didn't sound nor feel different.
However, I've also had a few moments where I thought "Aha! That feels different" with gear not as illustrious as a Dumble, so I'm pretty sure it wasn't the placebo effect. Tube-rolling was one of those times. The difference between a crappy Chinese generic power-tube and a GE metal-can and a NOS RCA glass was an ear-opener for me. Couldn't tell a difference in Tube-Rolling videos/recordings, but in person and playing them myself. Wow.
Placebo or not, if the player thinks/feels it's different and inspires them to play better, then that is all that matters. Well, so long as the overall tone isn't as bad as Henry Kaiser's — and let's face it, if that was Dumble's first amp then ol' Howie was just learning the ropes, teething problems.