I would first like to start out and say how I love what spotify is. This last year has been the best music experience(in my life) eaisly in the last 20 years for cost/collection size. Before to have acsess to this size library would cost $10,000's+
Now, with that said, and because of how innovative Spotify has been, I'm pleading to add a Bitcoin subscription option, with things like bitpay there is no worry about what a bitcoin is worth, the merchant can have fiat, bitcoin, or a mix of fiat/bitcoin. Heck you could even have a credit card still attached to the account, and a terms of service saying that you reserve the right to charge the credit card if the bitcoin trasaction was not valid for whatever reason.
As someone who for the first time in 15 years is paying for music because I feel the value is there, and also someone who has gotten countless people to join premium via word of mouth. You need to keep appealing to this target market, keep riding this wave of leading the industry. There is a massive potential for you right now, and I'm not sure you're taking it seriously.
Word of mouth is one of the most effective marketing tools out there you're obviously aware of this. The bitcoin community is quite loud, and very intrested in seeing it grow, with your help it can be a win win for both communities.
I hope some of the right people see this, either way thank you for your time, and thank you for such an amazing service.