Tim over at discchord.com has been offering beta testing services for a while now and it has proven to be really popular amongst developers. Sure you can usually recruit beta testers for free, but Tim has a deep understanding of music applications and synthesis. He really knows his stuff. Tim really works hard for you to make sure that your applications work the way users expect and are free of bugs. He already has a growing portfolio of clients including Moog, White Noise Audio and Distraub (check out the testimonials on his website).
You can hire Tim for a month for a very reasonable rate. Visit discchord.com for more information.