According to my schematic (I've noted the values I changed):

      volume   4M A (changed to 1M A)
      treble   250K L
      middle   25K L
      bass     1M A
      presence 25K L (changed to 5K L)

A = audio taper
L = linear taper

If you can't gig with your YBA-1 because of volume problems, try 
using your volume control. On all of my guitars, there is a 150k 
resistor and a .001uf cap across the input and wiper of the volume 
controls to keep them bright at lower volume settings. It sounds ten 
times better than using a Tube Screamer IMO.