Hope you're still enjoying your Ultrasonic Studio amp(s). I have here the following tubes on Odyssey:-
Cossor 53ku, Telefunken EL11 and RT EL39. My Sennheiser HD800 sound just like a top of the line headphone.
Still loving my Telly like it was new.
High impedance headphones really scale nicely with tube amps. Especially when you switch it to the high-impedance output of the amp.