[Shotimes] amp question

[van Oss]

The situation:
I would like to add a subwoofer to my 92 with Premium Sound.   I have been given a factory (which means JBL, I guess) subwoofer with
housing and amp.  Apparently, the main Premium Sound amplifier does not provide a connection for a subwoofer amplifier.  I have
watched several eBay auctions come and go offering the JBL main-and-sub amps as a package.  Budget is a concern.

The request:
Since budget is a concern, is my best bet to replace the Premium Sound amp with a JBL main amp?  Or is there an aftermarket main amp
that would connect to my Premium Sound wiring, but also support my wubwoofer and its amp?

Thanks.

Joseph van Oss