Are you looking for biggest possible volume gain with minimal distortion with a 9 volts supply, you have the Vallhagen Laudah. The pedal has one knob (for volume gain of course) and it is built in a mini box.
The designer’s presentation is as follows:
This my take on Jack Ormans Mosfet Booster project, the realization of his idea to create the cleanest boost possible. It offers a tremendous amount of transparent boost; with its gain control fully CW, it gives more than 30 dB(!). Despite its powerfulness, LAUDAH is a simple build. With just some few components around a mosfet transistor, it builds a one-stage gain circuit with high impedance input and low impedance output. LAUDAH fits in an A-size (”mini”) enclosure, and is designed with true bypass. If you are looking for one transparent boost, this pedal is highly recommended. Laudah amplifies around 30dB.
This pedal is also available as a DIY kit, Laudah kit.