Tough question. The real thing is quite rare and most people are not likely to ever play through one. But we do have a number of good references:
Some would leap immediately to a particular track, by a particular artist, like “Politician” by Robben Ford. Or maybe they’d focus on a particular album instead: Larry Carlton’s Last Nite - Live at the Baked Potato, or Sapphire Blue, Robben Ford and the Blue Line, or Talk to Your Daughter, or Sonny Landreth’s Grant Street, (if you want to know what a BluesMaster is supposed to sound like), or certain Carlos Santana tracks… the list goes on and on. There was a discussion on this very topic on ampgarage… see this post.
Problem is, live recordings excepted, we don’t really know what was used on most of the recordings, the possibility of re-amping tracks, the post-processing that was applied, etc. Compounding the problem, Mr. Dumble himself is known to tweek the amplifiers to suit the particular player’s needs and style.
One person that is renown for nailing the Dumble tone and sharing his experiments in tone shaping is Scott Lerner, a frequent poster on the forums, a.k.a. “DogEars” on AmpGarage. Scott keeps an excellent web-site with lots of his tracks available for listening… they are tagged with the amplifier he used: Scott Lerner Music. From time to time Scott will post direct links to experiments and works-in-progress on AmpGarage as he tweeks an amp to his ear and he will fill you in on microphone/room characteristics and effects/post-processing applied. He also frequently posts on The Gear Page, but the focus of those posts seems to be more oriented to the commercially available ‘Dumble inspired’ amplifiers out there. They are always worth a listen!