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counter-point, this being the internet and all, I've killed a number of in-ear earphones on the turbo due to sweat. Granted they were all really cheap rather than £300 B&O but at least 2 pairs were sold as waterproof sports ones. I've since been using some over-ear headphones which have survived a couple of years. They were also really cheap, the sound quality is terrible and they're starting to fall apart but the sweat hasn't gotten in to them.
Alas, the Sennheiser's are over ear and I sweat like a very sweaty person on the trainer. They'd be destroyed pretty quickly. But, I appreciate where you're coming from. Good causes don't miss out either.