My educated guess is that none of 1, 2, or 3 are correct, and that what really causes cap aging is thermal cycling. Krells tend to run hot (never owned one, but they have huge heatsinks for a reason), so this might shorten the life.
Leaving equipment off for a long period is probably OK, leaving it on for a long period is probably OK, but switching on and off every day is probably not so good due to the thermal cycling, which causes mechanical stresses (expansion and contraction).
The lifetime will also vary with the quality of the parts.
So if I were you I'd leave all solid state stuff on all the time to minimize temperature variations and condensation problems.
Another thing to bear in mind is that some manufacturers and repair houses make a lot of money swapping caps, and so it pays them to put fear into the audio community that their caps are bad and need changing. By analogy, try to find a car mechanic who'll tell you that oil really doesn't need to be changed every 3000 miles.
Leaving equipment off for a long period is probably OK, leaving it on for a long period is probably OK, but switching on and off every day is probably not so good due to the thermal cycling, which causes mechanical stresses (expansion and contraction).
The lifetime will also vary with the quality of the parts.
So if I were you I'd leave all solid state stuff on all the time to minimize temperature variations and condensation problems.
Another thing to bear in mind is that some manufacturers and repair houses make a lot of money swapping caps, and so it pays them to put fear into the audio community that their caps are bad and need changing. By analogy, try to find a car mechanic who'll tell you that oil really doesn't need to be changed every 3000 miles.