Here are some technical articles and more hard to find schematics.

Wurlitzer 200 Series Electric Piano

Here is the latest schematic for a tube preamp replacement.

(sorry for the poor looking schematic, I haven't figured out the best way to publishing schematics yet)

RCA Module information:

Check out the schematic for the BA-31C

Here is a picture of the BA-71C

And here is some literature on the BA-71B

Quad Eight schematics:

A simple 5 transistor discrete opamp design: MM-100 modules

Here is another discrete opamp, the AM-10

Nice inductor EQ:  MM-71 modules

(Those amps are thought to be an old video buffer relabled)

ADM schematics

Interesting output stage: ADM 668 module