As I recall. it involves resettting the leadpipe and a few other "tweaks" that Dave Fedderly has found improves the response of the 2145 and 2155, in particular. You might call; I doubt that it's a trade secret. Dave spoke very openly about it at the Midwest booth.
"The only problem with that tuba is, it does everything you tell it to!" - Robert LeBlanc
I tried to find out a few weeks back and he wouldn't tell me. I asked him "what if you get hit by a bus?" and he told me "Albert knows" (Albert works in the shop). The difference though (before and after), is impressive.