When to recap?

Sounds like you found an honest shop Good luck.

A ploy for income purposes??? That is baloney. It is true that many Mac solid state amps have G.E. computer grade electrolytics in the main power supply that can last quite a long time. The axial tubular caps in your Mac should be replaced. The rubber gaskets crack and the electrolyte in the caps dries up. Power output can decrease as well as having heatsink temperature issues. I never say anything like that to my customers.

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*!