Macintosh used very good quality caps in their amps, and there are many with original caps out there that are still running fine. Dynaco, on the other hand, used cheaper parts, and as a result, if the capacitor is the original, it's likely bad. Look for the date code on the cap. A typical example would be something like 6605, indicating it was built in the fifth week of 1966. Pretty old stuff.
New ones are tough to find.
Mail order is likely to be your only source. Don't overlook the bias supply capacitor. These are much easier to find, and rarely requires parts with more than 100v rating. Placing two capacitors of double the desired capacity in series that have about a 300 volt rating will work. Both caps should have a 100k 3 watt resistor in series as shown below.