My guess here is with tubes inserted in sockets it stressed the base of tube via pins being forced out and forcing base to crack over time. This stress with tempatures changes over years created enough stress to crack bases. Remove tubes when in storage.
I think it varies with tube construction and in my case, at least with the HA-2 - handling.
Case in point, back around 1993 I picked up a 32V3 that had been stored in an inactive hams basement for at least 15 years. From the day I took it over the glass covering the tuning dial was cracked as was the band scale drum.
Since Day 1 and as of last week, the 4D32 Final has played without a hitch with 100 watts carrier output on all bands. (hope it stays that way)
The construction differences between the 5894 and 4D32 are night and day.
.....WA1RNE