$500.00 headphone tube amp recommendations

Hi,

I'm looking to spend $500.00 give or take on a good headphone tube amp. I will be pairing it with ALO recabled Ultrasone HFi 780's, Denon D5000's, and ATH M-50's. I was thinking about stretching to get a Woo Audio WA6, but want to see what you thought. Thank you.

Thanks Skylab ! You're recommendations are more than welcome.