Thanks Steve / Allison for responding. Shock was the only thing that I could come up with as well.
After thinking about it, the HA-2 did take a hit at the power switch. It's been years but I recall jamming the switch knob against a shelf while moving it. Still not the kind of hit I would expect to cause a 2 1/2" diameter tube to crack, but that can be deceiving.
The HA-6 is a complete mystery. It was in an enclosed plastic storage tub and isolated from other gear for years.
Too bad, I wanted to keep those Hallicrafters tubes going as long as I could for old times sake.
The good news is, the first tube I picked up on eBay for $7.95 plus shipping provides around 65 watts output on CW which is about what it should be. I grabbed a second tube for $20 but haven't tested it yet, a Sylvania with hardly any wear marks on the pins. It's a crap shoot but guess I was feeling lucky.