I've had/have several early '60s ampegs, as well as a coupla Guild 66j. It's indisputable that Ampeg made most of the major components for the 66j--they're dead ringers in many respects, running the gamut from the chassis material to the silk screening on the control panel to many other major and minor components, including cabinet wood & design. But you're right, the Guild 66J is far more tweedlike than it is like an ampeg of similar vintage. for my money the 66J is an even more aggro circuit than the tweeds.
Every 66J I've ever had or heard or has been a raunch-inna-box, just full of beautifully ragged overdrive, they sound like they are on the verge of exploding (in a good way!), including the first one I heard decades ago that sent me on my quest to track down a couple. (It was played by an ol' blues dude that called it his 'secret weapon'). I occasionally hear of folks that say otherwise, but i'd guess that the ones they'd heard had been improperly modded--the original circuit is pure rockin rage.
The most reliable word among aficionados seems to be that Ampeg (which was just down the street from the Guild Hoboken plant at the time) jobbed out all the basic components, but the design & assembly was Guild's (& based on fenders).
Regardless, the 66J is stellar amp--let me know if/when you decide to sell it!
