I suspect that the aging process may have something to do with this. I have a number of very old globe tubes along with the newer equivalents with ST shape bottles. I consistently have found that the old globe tubes sound better. I doubt that the bottle shape makes a difference and I would be think that quality and manufacturing expertise would have been higher when the ST shape tubes were made (50-60's vs 30-40's).
In particular I have a set of blue glass Acurus 27's that were made prior to 1931. They sound notably better than the later 50's version.
In particular I have a set of blue glass Acurus 27's that were made prior to 1931. They sound notably better than the later 50's version.