Cutting bass and treble is the same as boosting mids, that's how a baxandall works... The Boss FA-1 uses 50k linear pots for bass and treble, it would be quite easy to replace both with a dual ganged pot. But it might require some adjustments, and maybe a better circuit already exists?
The Blade VSC preamp is more complex, but with a similar approach: a mini switch for t&b / off / mid boost, with three internal trim pots for the gain of each band. But that way, you only get two presets, none of them being completely linear.