Get the sound

This feedback has nothing to do with the annoying whistling. The NFB loop carries the signal from the speaker via a resistor back to the pre amp, usually the second triode or the phase inverter. This coupling should dampen the distortions and resonances. It makes the sound a little dull and cleaner. Depending on requirements, you can now retrofit it with a loop or modify the existing one or even remove it. A switch can be installed to utilize both states. An unproblematic and profitable modification.
The value of the resistor is based on the respective diagrams of Fender. If an anode electrolytic capacitor exists, the negative feedback must be applied differently, just take a look at the diagram of the Champ AA764.

If you want to filter out higher frequencies permanently, you can do this with the aid of a capacitor. To place a capacitor from the signal path to ground often has other effects on the circuit that we do not overlook. It is simpler, to lay the capacitor over an anode resistor of the corresponding triode (I like to take the second).

The capacitor must be voltage-proof, the anode resistor has a few volts. You can thus easily calculate the cut-off frequency of the filter because no additional components are in the game.

For a better understanding, I have drawn the condenser to the right of the resistor, because there it works - after the tube. For the AC voltage of the signal, it does not matter whether they are diverted to the positive side or to ground, the power supply electrolytic capacitors are a direct connection to ground for these frequencies.

The circuit of the input stage can be influenced by several ways.

Increasing the cathode resistor puts our operating point slightly to the left of the linear region. Thus we get a little unsymmatrical signal that with a sufficiently large amplitude becomes distorted one-sided. We keep the dynamic, but get a little saturation to the sound. For a 5751 the 2,2-2,7k have proven. This replacement, I would only do at V1, apart from that remain as linear as possible.

The cathode e-cap can become switchable, whereby you have two usable gain levels. The picture shows the luxury version with a resistor which prevents cracking noises.
Of course, this can be done for each cathode, but empirically the switch pays only for V1. Otherwise, it is enough to decide either for or against e-cap.