This  wikipedia page
Dumble amplifiers are extremely influental to musicians, amp builders/manufacturers, collectors etc. Their value is in their sound . As said on the wiki page many amp manufacturers are building amps based on mr. Dumbles design.The design is alive and available and influental.The man himself is a living legend.In many ways one could descripe Dumble amps superior, to many people they are, just ask Larry Carlton or robben Ford or Carlos Santana(famous musicians, if you dont know).All the business, either directly from the original amps or services from dumble, or in some way related to his work (similar products) or any sale mentioning his name is worth a considerable ammount of money.