It looks like on all my CBS Princeton schematics there's a 47 ohm
feedback scaling resistor attached to the 2700 ohm resistor coming
back from the speaker jacks. A 4.7 uF cap across this 47 ohm
resistor will boost everything above 690 Hz, a .47 uF will boost
everything above 6900 Hz - my feeling is you'll probably like
something in between, you might start with a 2.2 uF cap (negative
terminal to ground obviously if you use an electrolytic cap, 10 volt
rating should be fine). Experiment!