More than adequate, it sounds like. Purification Level 1 tones wouldn't be any lower than 100 Hz, and any commercial player can easily reproduce that.
I have never heard of headphones that have a frequency response that low -- are you sure its not 20 HZ? If not, I'm awfully impressed, although I don't think many homo sapiens can hear below 20. Maybe there's some indirect advantage to reproducing the lower frequencies which are imperceptible.
If you want to experiment with lower carrier frequencies to try to discover more of an effect, you can use Brainwave Generator, the website was mentioned in a previous post.
