When you say 'stock' Bob, do you know whether they've been serviced recently and when they were last powered up ? Also, what make/type are the output valves (they look like they could be Bentleys or Teonexes or Pinnacles) ?
The reasons I ask are:
a) if they've not been powered for a year or two (or more) then there is a risk of smoothing capacitor failure shortly after switch-on,
b) if there's any leakage in the inter-stage coupling capacitors (and there often is) then there is a risk of the KT66s going into thermal runaway (it's hard to tell from a photo but the left hand KT66 in the right hand amp looks like it's quite darkened - keep a very close eye on all of them for red-plating, and switch off straight away if you see that) and
c) if you want to make a judgement about the sound then you'll obviously want them to be doing what they're meant to, and if the passive components are still the original ones then the performance will almost certainly be way out of spec.
One other thing - if you think that their headroom sounds a bit limited then new rectifiers (GZ34s or 5AR4s instead of GZ32s) can make a significant improvement.