Actually, most bass knobs are a gain controller, and work quite well. The bass knob's maximum setting is whatever gain position you have set on the amp, and turning the knob down is the same as adjusting your gain down from where it is set.
In other words, the gain type bass knobs will only allow the bass to be turned up to where you have the gain(s) set, giving you control over bass output without 'boosting' the signal and adding distortion. This works well for systems that have overpowering subbass, it can be adjusted down to blend with the front stage of SQ listening, and it can be adjusted up for a good street beater session. While never allowing the amp's gains to go beyond where you have them set.
If your amp's bass knob is a gain adjustment type like I describe (good), or a bass 'booster' type (bad) depends on the actual amp being used.