Hello,
I found that the circuit of a non-budget headphone amp Felix Envy (7000 usd) is the same. I have little knowledge about tube circuitries, but can you tell me what output transformer is needed for heaphones with impedance 300-600 ohm? Do I need any changes in the circuit to operate the amplifier for high impedance headphones except for the output transformer?
I'm not a headphone user so have very little experience with them but you normally want the secondary impedance to match the load. Not sure you need a 300B tube to drive a pair of headphones. Like how many watts of power do headphones need? At some point I probably need to do a headphone amp build with a low power tube and some output transformers like these https://edcorusa.com/products/gxse5-5k-5w-5k-ohms-single-ended-tube-output-transformer?_pos=2&_fid=a2965e667&_ss=c
If you don't have much knowledge about tube circuits, you might want to start by building a well-established headphone amp that is specifically designed for high impedance headphones. I would suggest looking at the Bottlehead Crack 1.1 OTL headphone amp. It is perfect for 300-600 ohm headphones and can be purchased as a kit for under $500 USD. It is renowned for great sound and is an easy build for beginners.
If you don't want to do that, take a look at building the Budget 300B and also Skunkie's Headphone Box, which allows any tube amp's speaker outputs to be converted to headphone outputs. I don't know what the output impedance is, but Stephe probably covers that in her Youtube video on the headphone box. Good Luck!