[Arran]

There was not a valve tax as such, however during the 1920s, and up until the early 1930s, radio manufacturers were required to pay a license fee to the British Marconi company, and that was set based on how many valves the set had. The reason I read for why people preferred lower tube count sets is that members of the B.V.A, or British Valve Association, the tube manufacturers, would sell tubes to the radio manufacturers at near cost, and then soak the owners of the set on the cost of replacement tubes. On top of that they would not allow replacement tubes to be imported, so if you bought a set in the U.S, and brought it over, you would be stuck for replacements, especially with 150 ma tubes which none of the B.V.A members made.
 The radio manufacturers also used to collude with other parts manufacturers to keep new competitors out of the market by refusing to sell to them. This would not have worked so well with Philco since they were a huge company whom manufactured most of their own parts. It's a behavior that would have landed them in court in the U.S under the Sherman Antitrust Act for collusion, and conspiracy to defraud, and likely explains why they started to be left behind having to import technology under license.
Regards
Arran