The built in amp is worth keeping and with the monsoon outputs specific frequencies to each speaker.
Upper FT door speakers ( the small speakers) receive Hi frequency
Lower door receives Mid Range,
Rear 4 speakers on rear deck..
The 6X9 is the Bass and the smaller speaker next to the 6X9 is the Mid range speakers. If you buy anything other then component speakers they will not work properly because the speakers were only absorb the output frequency from the amplifier. If you use for our speakers it will make the sound diminish a little bit, Best to replace speakers in pairs and adjusting your levels to compensate for the 4 ohm speakers should he do this.
Personally I would replace the speakers with factory or upgrade using component speakers then add a dedicated subwoofer amplifier and subwoofer in the trunk. This would be the easiest fix to a good sounding system, I replaced the egg gu as well you will have to modify the 12 V (I used to relay) disconnected the power on from old wearing harness install a relay to power up the older amplifier using the new head unit amp on Power wire.
I retained the factory set up and it sounds awesome! With the added subwoofer and a subwoofer amp. Hope this helps