I would also bet that it's because of the competition. Marshall sells it cheaper in the US possibly because otherwise too few people would still buy/consider it.
One reason a lot of stuff is so expensive in Europe is NOT so much because of transport/duty etc., it is because people pay those prices, so they add a bit, increasing profit margins.
The same thing can be seen with cars. American customers are used to huge rebates on new cars and great deals. Car manufacturers have tried to get rid of those forever there, hardly with success. Similarly, the same Volkswagen car can often be bought cheaper in Spain than in the UK or Germany. It's even a problem with grey imports and reimports.
Another area you see this is also medicine. Lots of medicine is reimported from countries such as Greece and then sold in "more valuable" markets such as Germany.
So the morale might be never to question pricing policies - there's a lot of politics involved!
