Atma-Sphere has a 20-year design rule for the simple reason that 20 years is about as long as you can reasonably expect filter caps to hold up. After 20 years, even if they have not failed outright, it is a good idea to replace the filter caps anyway, regardless of the technology of the amplifier or preamp.
A friend of mine was a big Cello fan but one day the unit was making a thumping sound. I checked it out and a filter cap in the regulator had dried up. For guys like me the days when Cello was in business is not that long ago- so you can use that as a yardstick for what you are up against.
One further note- if the unit has sat in a basement or garage unused for more that a year the filter caps should be replaced regardless of the age. Caps don't take well to sitting around without a charge on them, much like a battery! So if the unit has been sitting a long time it should be regarded as suspect even if it is not that old.