Let me introduce the PIG, a germanium booster. Yes, it is what're thinking - based on the famous Rangemaster from 1960s. One of the best tools to push your tube amp into sweet overdrive tones, handy tone enhancer and shaping device for retro stuff purposes.
My version is transistorised with NOS OC140 germanium device, which lets you power it with standard negative center power supply together with rest of your pedalboard with no worries. And it still sounds kickass just like classic OC44 versions.
Level pot is simply a volume control, while depth increases low frequencies entering the circuit - so from a classic treble booster you can get into full range booster. This version is tuned to be clean with great presence and upper-midrange bite, also lowest frequencies rolled off to keep clarity and avoid overloading transistor. If needed, you can still get some dirty sounds with hot pickups or heavy playing, also turning depth pot to max yields in nasty punchy hairy tones which will pair with already gritting amp!