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In comparing the schematics of blackface Fender amps and silverface ones, I think the intent of the CBS engineers was to make the sound of the amp cleaner and more hi-fi. I haven't had the opportunity to look at many SF and BF amps, but it seems like SF amps have higher voltages on the plates of all the tubes. Another large difference can be seen in the phase inverter circuit.
In the BF circuit (above), the 1M resistors set part of the input impedance--the total input impedance is 2 to 5 times higher due to negative feedback from the "tail" resistor (see Randall Aikens' site for a great explanation). If you estimate the input impedance at 2M, with the 0.001µf cap a high-pass filter is formed whose frequency can be found with f = 1 / (2 pi RC), in this case 79hz.
In the SF circuit (above), the 1M resistors are replaced with 330K and the cap is now 0.01. This lowers the frequency to about 24hz using the same assumptions as above. This high pass filter lets more bass frequencies pass when compared to the BF values.
Changing the 470 to 680 changes the bias on the tube slightly. I don't know the effect of changing the 27K resistor to 22K.
I hooked up a DPDT switch on a SF Deluxe Reverb so I could quickly change between these sets of highpass filter values and did some blind listening tests. Both my wife and I agreed that the SF values caused a slightly "muffled" tone, while the BF values had better bass definition. It was subtle, so I don't know if I'd recognize it outside of this A/B context, but we both could hear the difference in comparison. This effect was most noticeable on the low E and A strings for the BF values. The SF values seemed to have better tone than the BF values on the D and G strings, but were muddy on the E and A.
Dan Torres (if I understand him correctly) says to change the SF 330K resistors to 1M to increase the input impedance, and I believe he leaves the cap alone. This would lower the frequency still further to 8hz which would seem to further enhance the SF muffled sound.
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