One way that you can use the 5k pot without getting scratchy noises is to do the following:

1)Reverse the OT primary leads on pins 3 of the output tubes
2)tie the tail resistor of the PI to ground at the same place in the amp where your OT secondary is grounded(should be back at the CT of the PT.
3)disconnect the grounded end of the middle pot from ground and tie it to the high end of the presence pot. Or to look at it another way tie the high end of the presence pot to the bottom of the middle control.
4)tie the feedback resistor to the bottom of the middle control.

This arrangement will have the feed back going into the grid fed side of the PI and will remove DC from your presence control. It gives a much smoother sound but the effect of the presence pot won't be as pronounced.  You can lower the value of the feedback resistor by about 25% to make up for this, but I usually find that this setup sounds good with the stock feedback resistor value.

Peter s