Tostitostelli
Head-Fier
Ah, ok.
OTL stands for output transformer less. It refers to the circuit design of particular tube amplifiers. Such amplifiers mostly work well with high impedance headphones, such as the Sennheisers and Aeolus in this case.
Bottlehead is a small company that sells kits for building tube amps. Assuming you can build one on your own, they give good value for money.
https://bottlehead.com/headphone-amplifier-kits/
Thanks for explaning ^_^