Elevate the heater supply to about +75v, with a voltage divider across the 300v supply. That brings both cathodes well within the usual +/-100v limits. A 220K and 68K resistor is close enough.
Bypass the 75v point to ground with a capacitor. I'd use an electrolytic of at least 10uF, bypassed with a ceramic of 0.01uF or greater. The large cap is to remove any common-mode hum from the filament winding, the small one is to take out radio frequency noise (the electrolytic has a relatively large parasitic inductance and looses effectiveness somewhere around 10kHz-100kHz).
To be extra-anal about it, center-tap the filament supply wit a pair of small resistors, 25 to 100 ohms each - or use the CT if the transformer winding has one.
Historical note - the first version of the Paramour had no bias on the 12AT7 heaters, which were in SRPP. I was counting on the leakage between cathode and heater to settle at an intermediate voltage. However, this contributed to a bit of a hum problem. Now I always ground the heater power, with a capacitor if it is biased and directly otherwise.
