-- ---
Can you scale your existing campaigns onto other traffic sources? That'd be my first recommendation - you're going to have a lot of experience in muscle that will really help you push it rather than starting afresh with a new vertical.
Also, have you tried direct media buys? I don't know the muscle vertical too well but I suspect there are a lot of large sites / forums / etc that would have a good match for the demo.
Can anyone with more muscle experience chime in?