Just to add a bit more information to the topic of debugging this input problem...
I spent some time working on a HD 400 recently (same guitar input circuitry). I attempted replacing the four caps mentioned above and also the opamp (U8). Neither solved the problem for me, so I did a bit more investigation and found that the input to the opamp from the guitar (through R58) was sitting at around -5.5 volts. After reverse engineering the circuit a bit more, this seemed to be caused by D31 - the high side (+7.5V) diode wasn't engaged and the voltage on the middle pin was almost the same as the low side (around -7V). I didn't have a replacement on hand, so I said a prayer and attempted just reflowing each of its pins. That seems to have fixed the input for me, works good as new now.
Also note, the only difference in the input circuits between HD 300 and 400 that I could see was that the component designators change. In the above, I'm referring to the HD 300 designators.
