first paragraph
"...with the M490 sound option, which means the addition of a 40 x 6 amp in the front boot"
The M490/M680 options prior to '03 only included a 4 channel amp. Since you have a '98 w/ door speakers, yours has the M490 option w/ a 4x40 amp - 40W output for each of 4 channels. OEM amp has a built in cross over for the doors (and is line level fed by combining signal from front and rear channels of head unit). If/when you unscrew it from the firewall, you'll also see a big white sticker on the front of it which tells you how many channels and what wattage per channel.
In a nutshell, the amp is ok but the drivers in the car are crap. You can change the amp or not but regardless you'll want to change the drivers. Especially the doors if you want any kind of lows. Oh yeah, and the dash ones too if you want any kind of decent mids and highs.
Start w/ the speakers but leave room in them for an upgraded amp if so desired.
You're welcome
