I was fortunate to talk with Kent Armstrong through the guys at WD Music. His father Dan Armstrong was Jack Bruce's techie. It is commonly said Dan put a diode in Jacks bass. Kent said it was a by-pass filter(capacitor) to drop off all the bass frequencies and the high frequencies. jack turned the mids all the up on his amps. Thw cabs used G-25's back then, G-30s used later in Marshalls. He regularly blew them and they distorted giving him his hard edged sound.
