The apparent real world reference used for the music hall Azusa visited in episode 9 of K-ON! has been discovered by intrepid otaku explorers, succeeding in unearthing the 3D world locales used in photographic references much as they did with previous expeditions.
Truly, nothing can hide from the all-seeing eye of 2ch.
The location is apparently a small music hall (of the sort so popular in Japan) named Mojo, and located somewhere in Kyoto, which makes it all the more likely that someone from Kyoto Animation might be familiar with the establishment.
Unfortunately for local otaku, stalking the music hall in real life is unlikely to lead to an encounter with anyone quite as moe as the actual twin-tailed guitar player: