Ive searched and searched and searched and searched but cant find info on how to pull the tower studs out of a dry subframe. I have generously applied pentrating oil to the bases of them, put two nuts on (to turn against each other) and used the biggest spanner I can find and nudda, nothing, not a movement.

Whatcha gotta do to remove these things. The ones that are in it have the structural integrity of cold soup (whch is probably better than warm soup but only slightly)

I can imagine snapping one is a PITA.

Basically what you've tried is what you do .... A little bit of heat on them might help as well if you've got a gas torch of some form as well but they are only a standard screw in stud ....

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If you have the subframe out, what I have done in the past is the same as your method except to provide a bit of extra leverage, I have used a pair of vice grips near the base of the bolt in conjunction with the spanner on the nuts and always managed to get them out. I must admit that I replace the bolts after subframe has been cleaned, painted and all bolt holes cleaned by running a tap through them.
cheers