When to recap?

The McIntosh tech's statement was a ploy, to get you to bring the amp to his facility. Electrolytic capacitor manufacturers, in their data sheets, will specify an expected lifespan(in hours) for their capacitors(during use), at a certain temperature. No one ever expects their capacitors to last 40 years(or even close). Aluminum electrolytics also exhibit a limited shelf life. Here are a couple of articles that you may find instructional: (http://www.emersonnetworkpower.com/documentation/en-us/brands/liebert/documents/white%20papers/sl-24630.pdf) (http://digital.library.unt.edu/ark:/67531/metadc3104/m2/1/high_res_d/thesis.pdf)

I should have qualified my statements by mentioning that
your
caps, in those 40 years, have gone through a combination of
those two dynamics. In other words; what would be
considered,
"time in service" and "shelf life",
assuming that the amp was not constantly powered up, or
completely unused, in those years. Thus; they will have
gone
through the dielectric deterioration(storage) and healing
process(during use), a multitude of times. Then there's the
factor of the electrolyte drying out, over those years.
Bottom line:
Replace your electrolytics(at least)! Your ears will thank
you.

Yes- its not a matter of which electrolytics in the power supply, its a matter of *change them all in the whole amp*!

I would also recommend replacing all of the electrolytics, I replace them whenever newer and improved models become available, from my experience the difference is staggering.