I just got back from one of the industrial places I do work for, and talked to one of their guys that is a plumber. He said these valves can be a real PIA when they don't come off easily, but didn't really have any hints.
Since you said a good plumber could remove the nut without cutting, can you tell me what the secret is? LOL
I soaked it for hours in WD40 and it would not budge.
I really would like to simply replace the guts of this one for now, as I plan to remodel this entire bath in a year or so. I will need a new bonnet nut at this point anyway, as I managed to dig through the chrome finish trying to get it loose. I hate to try and cut it since I am afraid of hitting the threads on the brass body that the rim of the bonnet nut connects to.
Any other tricks would be appreciated!!