Malo e lelei, utdeep!
Sounds excellent. I spent 3 months working at the hospital in Western Samoa in 1996 and made good friends with a guy from Wisconsin who was being a Peace Corper there. I also work with quite a few Tongan children and families in my work in hospitals in Auckland. They're lovely people - Enjoy yourself! (Yeah, they play pretty full-on rugby too!)
The North Eastern islands of Tonga have just been hit by a pretty nasty cyclone (it's that time of year), so you may well have your work cut out for you.
Aren'e you tempted to just grab a good portable CDP with a 240V adaptor and take the HD600s with you? Trust me, you'll appreciate the ability to sit back and relax to some music every now and then!
Good luck! Sai pea!
Hamsta