What needs to be doen to replace the stock amp and keep factory head unit?

1990 premium jbl...

I seen another post about this stuff, I understand that the factory amp input levels are weird or something... like somewhere between regular amp pre-out levels and below regualr radio levels... or something...

I am wondering what I can get to change the signal levels.

I could stand for the stereo to be a touch louder with windows down and roof open...


The previous owner has replaced the factory side speakers with "JL Audio" speakers, and I imagine I'd need to get a better than radio shack quality woffer for the sub box...

Thanks!

cheap adapter kit

The easy way is what I did a long time ago. You can get an adapter at walmart(I paid $7 a few years ago) that let's you plug an aftermarket head unit right into the factory jbl system. Then I ran the low-level outputs from the aftermarket unit to the aftermarket amp in the trunk. Sounds like a deal that's too good to be true, but it worked great for me. Good luck.