what guitar do you have? and why do you want to replace the pots? do you know what pots were initially in there?

There are really two ways you can do this - the simple way which is to pick one pot value, and the complicated way which is to add resistors to change their apparent value.

For the simple way, I would use a 250K volume pot and a 500K tone pot. Single coils generally sound a bit shrill with 500K volumes, and humbuckers usually don't sound too muddy with 250s. Using a 500K tone pot will let you put a tiny bit more sparkle back at the top end, without changing the basic loading. I would also probably use a .047uF cap not .022uF which can sound a bit nasal with single coils (and the JB is anyway).

For the complicated way, you need to use a 500K volume pot, and wire a 500K resistor (470K is the nearest 'prefered' value and will work fine) in parallel with each of the two "single coils" - this will give an effective load of 250K when one of them is selected. It will also give a load of 167K if both are selected together though... but if you're trying to get closer to a Strat sound that isn't a bad thing, because it helps to simulate the Strat's two tone controls operating together in the second position. You will also get the effective 250K value when using the JB with one of the other pickups, but this shouldn't cause any problem either.

If you're using a Strat-type selector switch you can also do it by using the second half of the switch to select the parallel resistor (since you only have one tone control you don't need to use it for that), so you can avoid the 167K load in position 2 if you find that too dull.

A treble-pass network on the volume pot will stop the true 500K value from taking away too much subtlety when you roll it down - if you use a cap and a resistor in parallel (probably about 220pF and 220K, but the exact values may need some experimenting) you can alter the taper of the pot without changing the value when it's full up.

Hope that isn't too confusing!

Basically, caps simply roll off top-end (or allow it to bypass the volume control if you're using that circuit, sometimes wrongly called a 'treble bleed', which maintains treble as you roll the volume down) whereas resistors change the loading on the pickup, which does also affect the tone, but in a more subtle way.

When the tone control is up full, it makes almost no detectable difference. As you roll it down, the larger value takes away more frequencies down into the midrange as well as the treble. It's often said that a larger value rolls off more treble - it doesn't, it's the upper mids which are different.

A .022uF can sound a bit middy and 'nasal' on single coils because it leaves more of the upper mids in, and the JB has that sort of voicing anyway, so I would tend to use a .047, it will sound deeper and softer. Unless of course you WANT a very middy tone! Vintage Strats actually used a .1uF which is even deeper sounding, but for some reason this value is now usually considered "too large" despite those vintage Strats being some of the most highly-regarded guitars ever made...

BlueAngel, you said resistors can be used on pups to modify tone. Could you please elaborate a little. Perhaps some practical applications.

It's really the same as using different volume pot values - the volume pot itself is a resistor in parallel with the pickup as well as a variable control. Lower values give a smoother, softer tone and higher ones give a brighter, more 'peaky' tone. By adding another resistor in parallel with either the volume control or the pickup itself, you effectively get a lower volume control value without having to change the pot or use a non-standard value.

You can also use resistors in parallel with either the upper or lower part of the volume control track (from the center terminal to one of the others) to alter the effective taper of the pot, but since this will also change the total value at least in some parts of the range, it needs to be done carefully.