When the amp is on and head unit is off, the amp can damage speakers or subs from any electrical signal. The amp can pop or thump, mostly depends on the wiring, amp, and installation.

 

since youre running your sub amp off your factory rear out id recomend getting an OEM bass reconstruction processor instead of a line out converter. it restores the bass frequencies that most factory head units take away. if you want outs for speakers and subs id recomend the mtx re-q

 

good call. AudioControl has some nice products as well. if you get a processor/eq that has high level inputs and RCA outputs you can gain some additional bass output through level or EQ.

the bass restoration products can counteract the weak bass output from factory systems. some factory head units will reduce bass output when the volume is increased in order to protect the factory speakers.