I never had any problems keeping my OEM 6x9's in the car as i thought they sounded fine so i put a sub in the boot!
but as you want the space you cant be doing that!
i think your best bet is to do a head unit upgrade, but £100 wont get you much!
i got my head unit from this place
Car Audio, car radio, car security, car navigation, in-car entertainment, GPS satnav, stereo, speakers, amplifiers, subwoofers @ In-Car Express the head unit was advertised in halfrauds for £399.99 in the sale was £299.99 i googled it and found that website, got it deliverd for £184.99!
still i think your 1st upgrade should be the head unit as the new 6x9's will need a decent player for them to work as they should!
hope this helps!