Crosby Spica
Mods / Crosby "Strads"
Perhaps the most well known mod of the period was the Crosby
Mod. Crosby was a "A High End
Audio Specialist" located in Hobbs, New Mexico. They
were well know for modifying a variety of gear, but perhaps best
known for their Quad ESL modifications... (more)
GR Research 
Replacement tweeter with matching crossover with
significantly upgraded components in a kit. They also offer components to upgrade your
existing crossover if you are just trying to replace aging
components. There has been much written in the boards about this tweeter
swap and the crossover kit and while it may produce a good
sounding retrofit, the considered opinion is that the finished
product will no longer be a time aligned design.
Basel Audio
- Building Classic Speakers
Technical review of the TC50
Tommy Tube - Pimping the Classics (also Spica related links)
Link to Spica Review, Audio Magazine 3/84