What is your music source? Why not use the source as the volume control.
My source is a Raspberry Pi computer with a DAC. The music is played through Logitech Media Server. I use an app on my iPhone called "iPeng" to control my music and volume. There is a free app as well called Squeezeox.
Why mod the input on the amp and possibly mess up the sound when your source probably has a remote control (i.e. CD player, computer, etc.)?
John