I only reach our customers via a few channels and mostly do content marketing but most of it is word of mouth.
We have not yet any referral or affiliate system in place but we are building one for our v3 which should come end of this month.
We had our first 10 subscribers with direct sales.
I guess the hard part would be to get on podcasts with a sizeable audience.
I've found that in my startup, even though it's small, it can be easy to get free marketing. I think many people assume that journalists have a backlog of articles and they ignore their emails with suggestions. And perhaps that's true for big ones. We got free press simply by searching the archives or a few local papers, finding a journalist that had written a similar article and reaching out through social media. They are paid to create content, therefore if you get in touch with easy to create content (you are happy to help write/edit, make yourself available at a moments notice) getting an article written about you isn't tough.