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A search on "hollow state" +tube will yield over 2 million google hits, and only a couple are for the brand or company you refer to which "appropriated" the generic term used universally today. There is even a published Hollow State Newsletter.
But adding "+tube" to the search would require one to know in advance that hollow state is a reference to valve technology. As a guitar player, I am very familiar with the virtues of tube-based amps. I have never seen the term "hollow state" used in any guitar amplifier company's literature, except for the "Hollow State" brand. I've never seen in mentioned in any music trade journal or publication or amplifier review.