What's up? This is Zach from The Music Gallery. I want to show you one of my favorite muting techniques to get that vintage P bass sound.
I'm using a Fender Elite P Bass with D'Addario Chrome flatwound strings. People used to keep a foam insert underneath the strings near the bridge to get the plunky muted sound. I play with mostly one finger in the left hand and used the other fingers to mute and modify the sound.
This gives flexibility between the closed, muted sound or the standard open sound. This makes it possible to put in cool legato fills against the plunky vintage mute style.