Thanks to amx05462 and Line6Tony for responding. AMX's comment re seeing somewhere that the Variax draws 800mA is the sort of thing I am after. I am working on the assumption that the rating on the factory supplied PSU has some headroom over and above the maximum needed to supply the guitar, not least because it also has to power whatever electronics are in the XPS box (either type) as well as the guitar itself.

It also has to deal with any power loss on the 25ft cable that comes with the instrument. I use a much shorter cable than this and will not need to allow for the XPS box in my intended application, so I am trying to arrange for a 9v (or whatever it is) supply from within the amp that will supply just enough current for the Variax & no more. Reason for this is that if the current drain is low enough, there may be sufficient spare capacity on the amp's transformer to supply this extra power feed. If this is so, it makes the mods to the amp relatively simple to do.