The 2000 SLE is standard with the Bose system as I recall. The Bose speakers are 2 ohm. If you are replacing them with 4 ohm speaker then you will have LESS volume, not more.
Also, the rear speakers are not the place you are concerned about crisp highs. The rears should only be contributing lows and midrange (although different people set up their system differently). Rear speakers are intended to fill in the rear to provide a three dimensional sound (as in a concert hall), most of the real music is in front. The rears are at a relatively low volume. Otherwise it will sound like the band is behind you.
In general though, OEM speakers are not usually that great (when compared to what is available after market).
2001 Bonneville SE - Dark Bronzemist Metallic - Mods: 180* thermostat, iPod adapter, Digital OnStar Upgrade
Planned Audio System upgrades: Front & Rear 7 Band Equalizer, Amps, Hands Free Bluetooth Cell Phone kit
Planned mods: Front PEM, Ported LIM, Ported Heads, ZZP SR Rockers, Aluminum Gaskets & Reprogram PCM