1) The "X" version had tone control potentiometers that were constructed to be electrically removed from the circuit when set to their flat position.
2) A 1uF electrolytic capacitor was placed in each channel’s tone control circuit to block the DC from reaching the audio output.
Whereas the owners of an earlier PAS-2 and PAS-3 preamps may not be able to find the specialized potentiometers that were used in the "X" version, they can and should add the 1uF capacitor to eliminate the DC Voltage in the output of the preamp. This is especially important if the amplifier that receives the signal from the preamp has a direct coupled input such as the Dynaco ST-70. The DC Voltage from the preamp will upset the bias point of the input stage of the amplifier and could cause playing issues.
The original PAS-3X used a 1uF, 25V electrolytic capacitor but it might be better to use a polypropylene or polyester capacitor with the same voltage rating or higher for the update. These capacitors may be found at Mouser Electronics. To add the capacitors just replace the wires connected between eyelet terminals #5 and #12 on the line amplifier circuit board and the lower terminals of the bass controls with the 1uF capacitors. Just remove each wire and replace it with a capacitor.