Quote Originally Posted by SLUDGE FACTORY View Post
When he was in KYUSS , he plugged his Les Paul straight into a bass amp for that dirty sound
Not sure about Kyuss, but for QOTSA the bass amp is part of it, but his main trick is using a Maestro Parametric Filter MPF-1 that was designed by Bob Moog back in the 1970's, which he has since been replaced by a Stone Deaf PDF-1 pedal (which is an enhanced copy of the MPF-1).