Should I spend time talking to 100+ potential customers before building, or just consult the handful I already work with?

My suggestion would be to connect (face to face if possible, Skype otherwise) with a couple from each category of users quickly (and multiple genres if that is an important category for differentiation) and then look to work towards the MVP,before getting more feedback (and ideally sales). Important to get that some of your initial consults are with users/customers outside of your network to ensure there isn't some kind of friendly bias.

I was part of a small team at Apple that in 1995 led our first efforts in what we then called the "interactive music" industry and helped explain to musicians, labels and other stakeholders how this "thing called the Internet" was going to change the way the music industry operates forever. I'm also married to a touring musician signed to a major label, so please take it from me:

Absolutely, talk to *at least* 100 customers before building anything and I would advise you (and anyone doing customer development) to rarely talk to people you already work with. They are more likely to want to appease you by telling you what they think you want to hear.

Building products and services for the music industry is one of the most difficult ventures to embark upon. I don't mean to discourage you (there is lots of need for continued innovation) but there are more smart people who have ultimately failed in attempting to service this sector than almost any other area of web and mobile based endeavors!

Happy to talk to you both about the customer development process as well as provide you some feedback on your idea itself. Best of luck either way!

Absolutely talk to your core of most important customers but then recruit a core beta testing group from them and use them for your detailed feedback and user research.

I just completed a three week beta test with the Guardian Newspaper on their new mobile app (I didn't like all of it) but they sent us a questionnaire a week and I tracked my use of it and the functionality/features.

Remember, in terms of new business development, those beta testers are in all likelihood your first customers for the new service - work them hard, make them feel special and then use their feedback and recommendations to launch and market the finished item.

Happy to amplify further if you tell me more about your business industry / service & product.