The Driver Stage.
The driver stage seems pretty uninteresting, tough at a second glance, you may be tempted to think that decoupling R10 with a capacitor of say 100mfd is a good idea! Or maybe that cap is not there because of current feedback? Two times no! This peculiar construction is the backbone of the Williamson amplifier. There is no feedback through this resistor and amplification will not be significantly increased by decoupling it. In fact there is very little ac activity over this resistor and yet it decreases the distortion in this circuit enormously.

The two tube systems seem to correct their unlinearities and the shortcomings of the previous stage. When you look at it any longer, this stage has some similarities with the so called "long tailed pair" phase splitter. Still it has its own short comings! One of them is the grid/plate capacitance of the 6SN7. At an amplification rate of aprox. 12 to 14 thanks to the Miller Effect it augments to 70pF on both inputs. 70pF on the cathode follower output doesn't mean a lot but on the plate side of the phase splitter it is significant. Though this stage tends to correct this mistake it is worth considering a tube with less grid/plate capacitance! The 12AU7 for example does the same job at less then half the capacitance! But the question is then, "is my Williamson still a Williamson?"

On the output side of this circuit the same problem arises, the Miller Effect of the output stage which is as large ( or larger ) as at the input side but the output impedance of this stage with R11 and R13 at 47kOhms it influence on the amplifier is much more significant and makes it hardly possible to make the amp run straight to 20.000cycles when no feedback is applied.

Since the input capacitance of a 12AU7 is less then half of a 6SN7 it seems a good idea to double the tubes like in the drawing above, at the same time reducing R11 and R13 by half ( think about their dissipation !) and enjoy the double bandwidth ! The good news is, "It works!" Alternatively you can drive two pairs of power tubes, reducing the necessary plate to plate impedance of the output transformer to 5000 Ohms, having twice the output power and still be close to the original Williamson time constants in the circuit. As long as you pay for the electrical energy it requires, you have my blessing.

More of a in between solution seems the use of a 12BH7 for this stage, reducing R11 and R13 to 33kOhms and adjusting the current through the circuit by R10 to 8mA. This solution sounds best.
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