Capacitor/resistor effect on audio signal
20-years old electrolytics, especially if they are only rated to 85 degrees Celsius have basically exceeded their lifespan. The electrolyte dries out over time. You should really tell us more about the original caps, but assuming a typical 2000-hour cap at 85 degrees C, you can calculate their expected life at 25 degrees C (average room temp.) as 2^((85-25)/10)*2000 = 128,000 hours = 14.6 years. (For every 10 degrees under the limit temperature, the lifespan [given at the limit temp] doubles). I actually have a 1985 electrolytic relic, originally rated at 6800uF (physically it is thick as size D battery), which has only about 10% of its original capacitance left. So drying out is quite real.

As for whether the audiophile-graded caps you bought are really worth the dough, you should compare them with new, run-of-the-mill electrolytics, not with the old ones.