Can you add a sub and amp to the 2001 1.8 T Bose system?

you have to use a speaker level adapter if you don't use another separate amp..
 

for your highs

Can't I just leave my highs and mids how they are, and just add a sub or two powered by a 2 ch amp?

the signal isn't simply run back on RCA. its a special wire mess.. you have to
 

disconnect the stock radio wiring, and replace it with the wiring harness in the link.

but you can use a speaker level adapter to run the sub and amp--

get a second opinion, i've never actually done this<ul><li><a href="https://forums.audiworld.com/a4/msgs/542493.phtml">https://forums.audiworld.com/a4/msgs/542493.phtml</a</li></ul>