if you have any free buttons on your controller why now just assign one of them to decrease the bass only and another button or modifier to increase it. That way you know that the one single button will always decrease the bass and alternatively when you want to bring it back up use can use another button or modifier to bring it up.
Even if you want to kill the bass completely and bring a new track in you can just assign a button to completely kill the low end (toggle mode so you can turn it on and off)…just make sure to turn it back on when your using the deck again
I’m not at home to look at the controller manager in traktor but possibly even setting a specifically value to turn the bass down when you press it once and then when you press it again it resets to zero.
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