hirasawa_susumu
A Japanese electronic musician usually known for composing music for Berserk, Paprika, Millennium Actress, and Paranoia Agent. He started the band known as P-MODEL in 1979.
K-On! makes references to P-MODEL through the five main characters' surnames. Many K-On! fan works make reference to this, with examples of fan works making references to Hirasawa Susumu using Hirasawa Yui found in posts such as post #481261 and post #480735.
He once made mention of Yui on his Twitter:
間違えてないか？私は平沢進だぞ。平沢唯じゃない。
Are you mistaken? I am Hirasawa Susumu, NOT Hirasawa Yui.