The BOSE system uses an amp, so should you. If you want to keep it hidden I suggest mounting the amp under the passenger seat or pull the panels behind the passenger seat and place the amp where the BOSE one was. You may want to consider removing the cover from whatever AMP you use and having a plastic or fiberglass vented one made. This will help reduce the overall footprint of the amp. This is especially helpful when mounting the amp in a hidden spot where it won't be seen anyway.
"The 300ZX Turbo is a dance; it's a song; it's rolling, roaring automotive art. There is no color that doesn't suit it. There is no mood-lifting chemical substance it can't replace." - Automobile Magazine