Installed bulk foil resistors in phono amp

Yes, I've heard good things about that tube.
A 10K grid stop resistor will give you considerably more RFI protection.

I've never really had any issues with RFI. Would the higher resistance not attenuate the signal? Or is it negligible compared to the input resistance of the tube? I see there is a 47k on the grid of the next stage. I'd repace a lot more but it's a very fiddly amp to work on.

It won’t attenuate the signal at all. Its not a matter of input resistance as it is input capacitance. This is also known as ’Miller effect’. The input stopping resistor does two things- it attenuates RFI by acting as a simple high frequency filter in conjunction with the input capacitance of the tube, and it prevents reflections from the signal coming in the grid and then ’bouncing back (’reflected’) to the input circuit. The latter can cause oscillation and instability, which can cause ticks and pops in a phono preamp. By increasing the stopping resistor value this latter issue is addressed.

But how high can you go? You can’t keep increasing the value hoping for more and more rejection! Maybe the designer deemed 1k enough.
The amp uses 2 diodes in series to bias the cathode. Can you suggest a better way to do this? I’ve seen NiCads and LEDs used.

@gs5556 Good to hear. 
The only manufacturer I know of that uses bulk foil resistors is VAC in their ref or sig series at $80k!