Managed to squeze twin 8"subs and 6x9s into mine. One set of power cables running twin amps in boot. Cables were very easy to fit using blanking gromit on bulkhead infront of heater motor. Sounds awesome. Highly recommended install. Will post pic when allowed lol.

total system is as follows,
non branded dvd head unit with 4 x 50w output,
front speakers, alpine type s in doors unamped, standard in dash.
rear shelf, sony xplod 6x9's powered by alpine 180w x 2 amp,
boot (most of it) twin JBL 8" subs in one box, powered by alpine 580w class d mono amp.
although i agree to an extent that it is nice to move the sound closer to the front of the car, the system as it stands is quite crisp and not chavvy in the bass dept.
i have transplanted this system through around 5 cars now over the years, although the head unit has not always stayed the same. out of all the ones i have fitted i found this the fastest to install out of all of them as the cable runs are already there waiting for you down the passenger side, and the unused blanking gronit (rh side rear bulkhead looking into the engine bay from the front) is ideal to pass power through into the cabin. you dont really have to amp your 6x9's but to be honest they sound so much better amped as thats what they were designed for in the first place.
hope this helps mate. prob the same advice others would give also i would imagine though.
Dan