Hiss in these amps is a combination of the negative feedback resistance being doubled (reduced), the bridging of the two channels to put reverb & trem on the "Custom" channel and the circuit layout. On my '68 CDR I returned the NFB resistance to AB763 spec, removed the jumper that bridges the channels, essentially making the "Custom" channel same as the "Normal" channel on a garden variety DR, and cut the trace to the V2 input from the volume pot and ran shielded cable from the pot to V2. To put the reverb back on the Custom channel I may do Rob Rob's mod, but I have to do some study.
This amp was the noisiest thing right out of the box. The grounding scheme is a bit nuts with loops everywhere. However you can make it work. I made quite a few bulletproofing changes, complete recap with quality capacitors (F&Ts), eliminated the 220/100 filter, uprated the filter & screen resistors to 3w, raised the screen resistors up off the board, moved the heater circuit off the PCB and mounted directly to the power tubes, Mounted the ACT near the PTY, etc. I triedf moveing the power section grounds off the board to points near the PT, but because Fender has the screen cap (also reverb & trem) grounded with the power supply, rtather than the rpreamp ala AB7643, it created a ground loop hum in the reverb. The only fix was to either cut the ground trac e on the main PCB or revert back to stock. Still a work in process, but quiet as a mouse now.
That is a lot of work to make a production amp ‘usable’, imho.