I would guess about 15 DBs, I used to have one, real pure tone boost, no OD, bass and treble controls, bought it fo 19 bucks in 05, Honestly, a real nice boost pedal, gave it away to someone down on their luck Jim
I love that pedal! The manual doesn't specify an answer to your question but it does make my amp much louder. It also adds a certain sparkle to the sound (as well as some nice OD). Incidentally the amp I use is a Behringer GM-110 (their version of the Trademark 30 by Tech21) and the pedal works great with it. So it is good for tube and solid state.