Skid, if you like the sound of your amp then leave it alone. Low B+ though may cause clipping at high sound levels and produce undesirable distortion if the signal clips. Modern 12AX7s can't handle more than 300V plate voltage tops so I shoot for 300Vdc at V2 pin 6 since it is connected directly to B+ supply. Most design aspects are a compromise so a few volts over or under is OK.I'm using a single 22K 2-watt decoupling resistor (power dropper at the NFB circuit.
With 22K PI, 8.2K V3-V2, and 10K V2-V1 B+ dropping resistors I would be concerned that the V1 plate voltage may droop from its already low value and take a while to recover since electrons are pushing through 40.2K ohms to charge the relatively large 33uF cap at V1. Plus the JTM50s are heavily filtered so you have other caps to charge as well down the B+ chain before V1. If you feel a need to experiment try 10K PI, 10K or 8.2K V3 to V2, and 2K V2-V1 dropping resistor values for the high B+ setting. You have a desirable B+ voltage of around 450V on the EL34 plates so it is matter of selecting dropping resistors for the earlier stages. If everything is linear on the low setting of 400V for EL34 plate voltage the preamp tube voltages should drop about 12% from the high setting.
Anyway, its all rock and roll and early breakup, grit may be very desirable. If you are happy do nothing. All the best, Scott