Wires carrying out of phase signals should be seperated. Gain usually means signal inversion. so as Limbe says, seperate input and output. A Cathode Follower has no gain and there is no inversion, no no need to worry about its input vs output. Remember is you have a lot of gain stages, The odd number are out of phase with all the even numbers.
Again, as limbe says twist the heater wires, shoot for 3 turns per inch
Don't forget grounds can be carrying large currents and so can radiate noise magneticly.
Don't use the chassis as a ground conductor. Only two things should be electricly connected to the chassis, the incoming safety ground wire, and 1 connection to the ground network, preferbly at the input jack