Circuit Research Labs, or CRL, as everyone calls it, was in my backyard of Tempe. Arizona back then.
Ron Jones (sk) owned the company, and with a crack team produced some of the most successful audio processors ever built.
The APP Audio Preparation Processor (an agc unit), the SEP Spectral Energy Processor (a multiband EQ unit), and the PMC Peak Modulation Controller (a tough limiter) made up a complete set that was used to beat boxes like the Optimod in many AM broadcast installations.
If you are lucky enough to find the other two units, you would then have a system that could not be beat for clarity, transparency, and punch. They were designed to work as an integrated system.
It may not come without an electrical cost: A station I was CE for back in '83 (KZZP) ran a vintage Bauer FB-5000 transmitter that sounded just fine, but the program director (read licensed psychotic) wanted it to dominate the band and sound very aggressive.
I called Ron and he came over and made enough measurements to determine what was necessary to make that TX produce extreme modulation density without ever peaking 100% neg.
A custom package including a one-off tilt correction box was installed. Man, what a difference it made on 1310 in Phoenix!
It all came to an end three months later when the boss found out a pair of 4-1000 modulators lasted all of two weeks before being just burnt flat dead.
The old box was put back as it was. $$ ruled.
Experiment with your box to determine where the 'happy' spot is and don't push it too far...it can sound great without frying tubes for the sake of a couple dB loudness.
All the manuals are available online now from Orban , as the current owners of the CRL line:
http://ftp://ftp.orban.com/CRL/73DG