This is MXR's take on the classic Tube Screamer circuit. The 'bump' switch toggles the mid boost on and off, and the 100hz knob adds extra bass as you wind it up. Sounds great, it talks a lot of sense.

Model: MXR M77 Custom Badass Modified OD 
Made: USA, recent model
Type: overdrive, TS style
Power: standard 9v negative supply (not included)
Switching: true bypass
Equivalent new street price (approx): $189aud
Mods: velcro attached
Accessories: original box and manual
Condition notes: used, no major marks, excellent condition