Quote Originally Posted by blinddrew View Post
Hmmm, turns out I'm not quite done yet...
When i turn up the tone pot (i.e. to a brighter setting) I'm getting a lot of noise, which disappears completely when the knob is at its other extreme. Any suggestions?
Well, your tone control when set to 0 is filtering off all the HF, so it's not surprising the noise disappears. But it means that it's general RFI, and not mains hum, which is good. You'd still hear and substantial 50Hz noise with the tone control down.

P90s with their wide coil do pick up a lot more noise compared to say a Fender single coil. Have you checked that the strings are still grounded?

Of course you could swap to a P90 with a nickel cover (avoid other often steel covers as they seriously affect the sound). I'm sure I looked round and you can get black painted nickel covers (to keep the same vibe) and one of those should cut out most of the noise. But the cover needs to go right down to the base plate of the pickup so it can be grounded, so again you've got the problem of matching a cover to the height of the dog ear arms. You can do a certain amount of bending of the arms to get the height right, but you don't want to do it too much so that the metal ears aren't supported by the body or stick out under the plastic.