Minor quibble, but the quad adjustment is peaked on the center frequency, not offset as in slope detection.
As the tube ages, sometimes you have to re-peak the quad coil and "buzz" pot for best sound.
Excellent tutorial on gated-beam detectors here..
http://www.tpub.com/neets/book12/51e.htm
Since the tube (usually 6BN6) plate is supplied from B Boost, the detected audio is high level and drives the output stage directly. Completely eliminates need for a 1st audio stage.