In a time long ago, 2001 to be exact, Keeley’s mods for the TS9 Tube Screamer became pretty legendary. They set the standard for a complete set of changes, a dozen in fact, that took any screamer to the next level. Those mods included an increased bass response, a fine-tuned tone control, greater cleans when turned down, and much higher gain when needed. The Keeley Mod Plus was born.
source: robertkeeley.com