As mentioned here, the sub is going to provide a more balanced sound. If the speakers are high passed at 80hz, (and there aren't many speakers that will play down to 80Hz linearly), then there are 2 whole octaves of music missing. The sub will flesh out these 2 octaves and give back that oomph.

Where it all goes wrong, is working out how much extra 'oomph' you need. Whacking 2 x 12's in the back is not going to provide a very balanced sound. For example, you are listening to music that is reproducing a kick drum. Typically a kick drum will have fundamentals that are reproduced by the sub. But if you turn your front speakers off, that kickdrum suddenly sounds muddy or boomy. So you need the midbass to give it that snap, that punch in the chest feeling that a kick drum should have. So now you need good midbass speakers. Remember before I was saying that most midbass won't play linearly down to 80hz? Having 2 x 12" subs booming away at 80Hz and under, and then a piddly 5" oem driver playing from 80hz up, is going to result in a pretty muddy and boomy sounding system. So, choose carefully, a quality 8" or 10" ($300-$400) will give plenty of bass to accentuate the factory system without having massive amounts of boominess, or that 'cheap' sounding $150 pioneer woofer sound that so many people think is great.

Good luck.

Orion gear is good, but im not sure if Autobarn only get the basic range?? Orion used to make some quality loud gear.

Go the kickers mate, I just put an L7 12' in my sedan and fmd it blows all other 12's for $600 and under to hell. But be warned, the kicker is 310mm high, technically a 13' sub, not a 12'.... and the parcel shelf to floor is only 280mm high, so have fun designing some angles on your box!! it took alot of farting around and 3L of fibreglass resin to get my box going as we had no choice but to use a thin base for the box to reduce height, also the subs basket hits the bottom of the box due to the angle!! ALOT of stuffing around!!

guys ..I'm going to say spaekers first. The stock speakers on my premium bf aren't that great (don'y know about the FG). I bought some entry level mb front splits with an amp....HUGE difference in quality. I added the sub later on.

I'd just save up the money until I could do it all at once

Nothing in the "premium" system is great. it's a matter of which is worse. You'd think that ford would build something a little better into our local cars. The imported cars have so much better stuff standard

I went subs first. Well had the gear from my old car.. 2 12" Alpine Type R's, Monoblock amp and a 4 way for the speakers.. Im happy with how the std speakers are holding up.. This is through a headunit.

I spoke to a guy from JB Hi Fi that had been installing car stereo for 30 years. He was a bit of an arse actually and basically hung it on everything. But his advice sounded pretty good.

He said, as listed above, the backs are there for the rear seat passengers. So stuff them. He asked what I hoped to acheive and I just said better quality.

He said the best thing to do is put a sub and amp in. Then you can wind back the bass from the ICC and overall sound will improve as distortion will be less, plus you get the oomph.

He recomended a few but I never listen to them when they go the sale.

But its in line with what most of you say.

My daughter had those MB quartz splits in her honda and they sounded the goods. I bought them for her for christmas a couple of years back. She stacked the car and now they are sitting my garage.

MMM, time to get the screw driver out. She'll never know. lol

If you get really good front speakers the back seat passengers will enjoy them just fine. Too many high frequency speakers mean a loss of direction of the sound.

Could not stand the 'premium' sound at all went to CAE and he suggested Orion xtr's for fronts and put my 2 10's boxed in the boot an run off a 4 channel then if I wanted more power add in another amp but it's more then enough and for 700 I stalled for splits an amp I'm happy as