C300 is in parallel with C301. They are the input voltage filter caps.
The fact that you run well without that cap only means that the other caps on that same circuit have picked up the slack....but also that they are working harder, which means premature failure. Of course, you know that the caps are prone to that anyway, so no news there. Also, when you use the CD with the TG16 interface, the caps and power supply in the interface are helping to support proper operation.
Charlie