First, in the Smithsonian interview, he said "80-90, 89 volts is the sweet spot" having heard Ed interviews, I would bet he meant 80-90v and just mumbled and said 89v but still great insight.
Second, on the Letterman Show, I can't imagine that they would have him using his amp. A TV studio is no where near the size of Gazzari's, It would have had to been at 40 volts. Maybe it is more for a prop?

But then again, why would he schlep his #1 head and cabinet for a prop??
Then again, it is mic'd so the prop theory seems not likely.
So, not much to add being snowed in for two days with kids climbing the walls.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.