The 98 w/ monsoon and 02 SS use the same drivers. Such an upgrade would be pointless.
If you want better sound start with aftermarket speakers, then the head unit, then the amp, adding in subs wherever in the list makes sense to you.
Dropping in an aftermarket deck and speakers, using the monsoon amp can sound very good if you take the time to understand how to wire everything. If you're mostly looking for bass then adding in a simple bazooka or similar self amped tube offers good value.
2000 Z28 MRM/Tan Convertible A4
!HS (founder), SLP Lid, Tamed Corsa Catback, HP Tuners, 3.42's, TA Cover, Koni's
Cheap 17" Z06's w/ Fuzions, Stull Grille w/'69 Badge, Custom VHT Tails, Stainless letter inserts, Black Halos
JBSlim 4x 8" sub box w/ RF subs, Kenwood HU, Alpine & Infinity Drivers, Memphis Power, and a touch of Dynamat X