I have a 1998 A4 with a Borla cat-back exhaust.

it sounds good but i was wondering what would make it more of a deeper tone?

 

Borla is usually about the lowest tone you can find. Do you have the 1.8t or the 2.8?

Another low pitched exhaust is Remus. Try youtubing around to find the type you like.

About half way down this page youll see a range of exhaust brands and sound bites for each one:
https://www.audiforums.com/forum/audi-a4-8/newbie-guide-version-2-a-80968/

 

Well I was thinking replace the resonator with a test pipe or get an exhaust manifold. Mine is a 1.8t. Idk I need some tips. I'm not an exhaust pro.

 

ecs test pipe 100 had it on my car for a month now crashed.junk