a) There are several iterations of the PAS. The PAS-3 version is referred to by the OP. The 2 and the pre-mod 3 *will* be challenged driving the two amps together. At low volumes, this might not be an issue.
b) The PAS-3X will not be challenged by this option. The tone-controls are 100% out-of-circuit when in the null position, and the output impedance has been changed from the older versions.
c) It is possible to to the X-mod to a 2 or 3 as it applies to output impedance, but not to the tone controls. This will help in most cases and possibly be all that you need.
Per Dynaco, as follows:
1. Obtain two 100,000 ohm, 1/2-watt 10% tolerance resistors.
2. Unsolder and discard the two 510,000 ohm (green-brown-yellow)
resistors connected to the audio-output sockets on the back panel of
the preamplifier and replace them with the 100,000 ohm resistors.
Solder all connections.
3. Snip out and discard the two 62,000 ohm (blue-red-orange) resistors
on the PC-5 circuit board adjacent to eyelets 8 & 18. This completes
the modification.Given the relative difference in cost, I would use 5% resistors and
screen them as well.
As to how you can tell if you have the X tone-controls:
1. with your VOM go to either element terminal and the wiper.
2. Operate the control through its range. At/near the center-point
there should be a signficant drop or a significant increase in
resistance. If that happens, you have the X controls. If the
resistance change is approximately linear to the control and with no
change at the center-point you have the pre-X controls.
But, for the record, I very much dislike mutilating this pre-amp for dubious results easily corrected otherwise. I look at it in almost the same light a declawing a cat. Or docking a dog's tail.