As Jay rightly says - you can't connect your sub pre-out directly to a passive speaker - the pre-out has to go to an amp which can then be connected to a speaker, or to an active sub. I was doing this job on the cheap and don't have a sub pre-out on my head unit so just used a spare rear channel as i only had two front speakers originally. Fortunately it worked out nice! If I had connected an amped sub I would then have to had to buy more powerful mids and tweeters to match all that bass. My head unit is a 4x45W kenwood and its plenty loud enough for me with it turned up to around 60% power. Beyond that the little speakers in the dash start distorting. Like I said at the start of this thread, this was meant to be an 'optimal bang for the buck' system...