id start with the simplest thing first, try a better ground the the converter. the thud you hear from the sub when the radio turns on is normal, some amps like my JBL BP300.1 dont have this problem, it has a silent turn on circuit. The messed up thing about that is it can cause problems with your sub is exposed the that for a long period of time. im not too sure about the feedback you're getting with the radio turned off, that's a first by me. I hope you figure it out.