System upgrade

[TH3H05T]

Hi, I've got a rev5 mr2 with I think the active sound system? the storage behind the passenger seat doesn't open so I'm assuming it's in there lol.
anyway i'm looking to upgrade the speakers throughout but i'm not sure about needing to replace the amp?
I was thinking to get a sub for the back but I have a T-top and the sub box on mr2-ben takes up the space behind the seat.
also any recommendation on specific hardware would be greatly appreciated