Hi I have a Very Good product for your problem. I also had this same problem when i lost the bass knob to my amplifier So I bought a Ct sounds bass Knob it is very effective and also comes with a built in volt meter. Italso looks very clean here is the link to the product if interested http://thecaraudioworld.com/product/ct-sounds-universal-bass-knob-with-digital-volt-meter/#tab-reviews
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=== Update (06/02/2017) ===
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Here is another Good bass knob I found hope it helps. http://thecaraudioworld.com/product/pac-lc-1-remote-amplifier-level-controller/#tab-reviews
Hi I have a Very Good product for your problem. I also had this same problem when i lost the bass knob to my amplifier So I bought a Ct sounds bass Knob it is very effective and also comes with a built in volt meter. Italso looks very clean here is the link to the product if interested http://thecaraudioworld.com/product/ct-sounds-universal-bass-knob-with-digital-volt-meter/#tab-reviews