As an amateur euphonium player, how can I continue my studies in a scientific and systematic way?

[AmoHidestar]

This is my third year playing the euphonium. I can play from low F (three ledger lines below the staff) up to high E (three ledger lines above the staff) on my 4-valve euphonium, and I can do single tonguing at 220 BPM. I’ll probably keep playing for a long time—who knows?

Could anyone recommend some methods, etude books, or study materials to help me keep improving my playing technique? Thank you very much!

[circusboy]

It sounds like you've already been working scientifically and have developed solid technical skills. I'd recommend working on the opposite for a while: having an emotional connection to what you're playing, having fun, creating beauty.

You've been working it, which is great. Now's the time to start playing it.

[tclements]

Take lessons with a pro player ....