Not familiar with Tonedevil Harp Guitars but you may want to contact these folks - there are a lot of resources related to harp guitars so I would imagine someone there would likely know something.
http://www.harpguitars.net/hgg/hgg.htm
Very cool that you want to learn the harp guitar. They are amazing Instruments. I understand that some of the challenge is in not only learning how to play the subs but also be able to mute them.
Best,
Jayne