See that 4700 pF cap across the 470K grid resistor? If this was not a module with those resistors and caps, one could in essence remove the 4700 and replace it with a smaller version to get more high end audio through. Unless you feel daring enough to buy the components and remove the module altogether.

Tha 4700 pF cap is there to filter out whines and whistles of RF interference. Experiment with the value until you get the best balance. This is likely true also with the solid state variety.

I thought that the 200 pF was a neutralizer to eliminate oscillation due to phase shift????

The separate diode idea is always an excellent choice.

I have replaced the tube rectifiers with solid state (germanium) diodes and got excellent results in both fidelity and sensitivity. Why do you suggest changing a SS one with a tube?

One can also detune the IF stages to get a wider audio response, since too high a Q narrows bandwidth if you so dare. But first mark the original setting with magic marker, pencil or nail polish, and don't go nuts with the screwdriver or you can break the lugs, and go overboard with turning them and forget where they were originally. One loses some sensitivity and selectivity, but those are the trade offs for sound quality. This is true for both technologies.

BTW, 5KHz is the highest note that is considered musical anyway ("The Science of Music" & "The Psychology of Music". Good reading).

Gabe