Electronic – valve tone control circuit build

Let's start with hum. If your heater supply is rectified, then 50Hz hum usually gets inside your audio tract via capacitive coupling from the power lines (all the wiring in and outside of your house is basically a huge antenna). If not, see below.

Also, a TV sync signal somehow detected inside your amp can present itself as a slightly "phasing" hum. It is not immediately obvious what comes before the preamp in your schematic, but the lack of any series resistance with the grids usually invites radio interference. The 100Hz hum usually comes from your power supply via a number of common means:

1. Insufficient ripple filtering. Can be mitigated by adding a choke (if not present) and/or enlarging filtering capacitors.
2. Magnetic interference directly from the power tranny. Some power transformers operate near their core saturation intuctance levels and therefore can in some cases (not ideal sine in the mains supply, etc) have heightened inductive leakage which can affect sensitive low-voltage circuits. This can usually be hinted at by a hot transformer and distorted sine waveforms on its secondaries.
3. Heater supply. If your heaters are not rectified, you can experience 50Hz leakage onto the cathodes. This can be diagnosed by adding a switch to the heater supply and switching it off while the amplifier is operating. If the heater supply is the culprit, the hum should go away immediately. Rectified heater supply can also induce this kind of hum, but on 100Hz. If this is the case, you can consider elevating your heaters by a DC bias and therefore saturate the leakage current so it does not fluctuate anymore.
4. Grounding errors. Ground loops, insufficient grounding of high-current parts of the device etc. can cause parasitic voltage drops inside sensitive parts of the tract. There is usually no universal solutions other than following grounding rules (see "star grounding" and "bus grounding") I usually use a mix of the two.
5. Lead dress. Having pulsating current loops inside your device near sensitive circuits can couple hum into control wires. Usually diagnosed by identifying potential offending conductors and physically fudging them about listening to changes in hum. If this is the case - you can permanently move those away from sensitive circuits, twist loops in pairs etc.

Crackling in the pots can be a sign of DC voltage present on them. I'd carefully measure if there is any DC on any of the leafs which can indicate a faulty blocking cap. If this is not the case, I'd assume faulty pots as well.

As for the pot resistance. It is hard for me to judge the effect, so I'll give you these network analysis plots - the first one is plotted using the original values, the second one uses 100k for treble and 250k for bass. I'll leave it up to you to decide if this is appropriate. Also keep in mind that you will load down the previous stage more if you use smaller values for the pots.

Cheers.