> the junctions of R1-R4 and R17-R20 should be very well-bypassed.
No. The gain from that grid is 100K/Rp(V2). The main source of rail hum is V1a's high effective plate resistance (due to the V2 plate in its cathode) allowing ALL the rail hum to appear at the output R11.
This is a "fault" of this amp compared to a classic common cathode input. The conventional way gives 6dB-10dB power supply rejection, this way gives 1dB-2dB PSRR. Not a serious DIY fault because we can afford more than the 10uFd-20uFd that Fisher and folks used in phono power filters. But don't forget that there were usually 3, 4, even 5 stages of R-C decoupling from the rectifier to the phono stage, and it was not uncommon to lose 70+% of the original B+ along the way to the phono.
Lessee... ripple at the first PS cap may be 2% or 6V. Ripple referenced to the input should be a microvolt or less. Ripple at the first stage plate can be ~50X higher, 50uV. 6V/50uV= 100dB ripple reduction!!!
An old commercial rule of thumb is to take 20dB per R-C stage: to get 40dB, it is cheaper to use two 20dB stages than one 40dB stage. That says 5 stages (C)-R-C-R-C-R-C-R-C-R-C. Fat asian caps are cheap, we might go 30dB/stage for 4 stages, or 33dB/stage for 3 stages.
The hacked plan shows a 290V dirty rail and a 185V rail that must be clean and supply 10mA. (290V-185V)/10mA is 100V/10mA is 10K total dropping resistor. Say three 3.3K resistors. To get 34dB in each stage the cap reactance must be 3300/50= 66Ω at 100/120Hz. 24uFd, which really is not bad.
Duncan's PSD, for 60Hz 200VAC, FWB rect, 25uFd-3.3K-25uFd-3.3K-25uFd-3.3K-25uFd and 18K load, says 179V final output and 8.8uV p-p ripple. That's hardly an obscene cost.
Changing to 250uFd-10K-250uFd gives 129uV ripple, which shows the folly of trying to do it all in one stage. A couple 470uFd caps around a 10K resistor might work.
100uFd-5K-100uFd-5K-100uFd gives ~4uV and at today's cap prices might be the sweet-spot. This goes up to 7uV when I tap 10mA (for the CFs) from the first cap. How-ever... that's 1.4V of ripple at the CF plate, and 12AU7 CF ripple rejection won't be more than ~20dB. Even dropping another 1K-100uFd in there gives 15mV at CF plate, over 1mV of ripple at the output jack, just too much.
Yes, power supply cleanliness IS a problem for a hungry lo-PSRR phono preamp.