I had basically this same issue with a Beta Bass I just worked on. A fault on the -15V line killed both IC103 and IC106, so replacing them got it up and running, but I was having the issue of the LEDs staying on after unplugging. The JFET switch would remain on too - you could hear hiss through the preamp until the light turned off.
C165 and C164 were soldered between pins 1/2 and 5/6 on IC103 on the back of the preamp board - I believe this was a late addition to the circuit, so it's possible that not all versions had it. You can see them on the schematic you posted. When I replaced IC103, I had removed them, and I neglected to put them back. I reinstalled them and the problem disappeared.
Hope this helps!