The first one which comes to mind is the plate load of the grounded cathode amplifier( lower section, fed from the volume pot ).

The next mod is, can we do away with the resistor between the two output sections? or, has anybody experimented with putting the output at both ends of the resistor?

Also, changing the performance of the first half's PSRR will require a different application of PS noise to the second half. Either with a different pair of resistors, or a pot.

By my own experience, larger output caps would probably help. It depends on the rest of the system. I like 7.5-10k grid resistors in my amps, so the linestage's ability to drive a SS-like load is required.
regards,
Douglas

Smaller cathode resistor gets less impact to output Z w/o breaking the original circuit performance.

Add to this, most of the amps I have build don't need much gain from the linestage and we are into 12B4's for the amplifier half....
cheers,
Douglas