Here is an interesting device from the early eighties. Its a passive bass boost which connects inline between your guitar and amp.
No batteries, no switches, no knobs…but it works! This thing has been sitting around in a guitar case for years. I never really used it for guitar but I remember my bass player (DO) loved using this thing. It is completely sealed and potted so there is no way to open it up without destroying it. I am pretty sure it uses a small step-up transformer to achieve a small increase in level along with some treble cut to produce the effect. Zeus Audio also made some other devices you can find around but they are pretty rare. There was a straight Booster like this one and they also made a small practice amp and a preamp/distortion device both of which were active and used a 9V battery.
nonLinear 