Okay, Liam - sorry to burst your bubble here, but those figures you are quoting are the "MAX" figures. They're - for all intents and purposes - completely meaningless.
You need to look at the RMS power of the amplifiers to give you an idea of how much power they can really put out - and sadly, Sony kit is well known in the industry for being horribly overrated - I've seen mono amps rated at 1500w MAX give out less than 300w RMS before.
It's not perfect, but a good way of working out the true system power is to look at the blade fuses in each amplifier.
If an amp has 1x40A fuse, then you can derive its maximum output power - P=WxV, so 14.4v x 40A = 576w.
That is assuming the amp is 100% efficient - which it isn't - so the most you'll see from that amp is probably 450w RMS. As you can see, this is a fair way off from the 1500W quoted on the box.
Personally, I wouldn't run any more subs. I also wouldn't run the 6x9s.
In actual fact, you're probably only giving each sub about 250w RMS, so instead of spending the money on another Sony amp and more subs, you'll have much
better results from upgrading the 1000W MAX Sony amplifier to something that can actually put out some real RMS power. What is the RMS rating for the subs you're using? What
configuration are they? Basically you want an amplifier that will do about 10-20% more power than the subs are rated at, so you can correctly set the gains and give them plenty of clean, unclipped power.
The 6x9s - bad idea, especially with a pair of 12s. If you've got these mounted in a parcel shelf, then you will find that the movement of the woofer cones creates pressure in the boot - which is exactly what a speaker is supposed to do. The only problem is that you've got two 12" diameter cones moving together...bit of maths tells us that the cone area of both woofers (A=pi x R-squared) is (2x (3.141 x 36)) - 226 square inches. That's a fair amount of cone area, and when you compare it to a set of 6x9s, you'll see that there is a very real chance that the movement of air in the boot will force the smaller oval speakers to over-excurt, tearing the spider and damaging the speakers.
If it was me, I'd ditch the 6x9s, and just leave the holes there (cover with acoustic cloth or something) and then spend the money on a set fo component speakers (or coax if you prefer) in the front of the car, run from the amplifier you were using to run the 6x9s.
This will solve a lot of problems - you'll have a MUCH better sound stage, as you'll have louder, clearer, more controlled music coming from up front, and this will integrate much more tidily with the bass from the subs.
Please be careful setting the gains, and PLEASE make sure you fuse EVERY SINGLE power connection to every amplifier.
If you want any advice, feel free to ask me. I'm not being harsh for the sake of it, but because I run a SQ car myself (well, I did until I stripped it last night!) and I genuinely get pleasure from letting people sit inside it and play their favourite CD to see how much difference a bit of thought can make to someone's install - remember the old adage - 40% kit, 60% installation.
For everyone else on here - using a sub or two doesn't mean it has to be deafeningly loud. There's a discipline within car audio called "SPL" which does aim to get the most noise inside a car, but the important thing to realise is that people don't actually sit inside the car when this takes place! My Laguna - when I had it - was capable of over 132dB with a pair of 12" Diamond woofers in a three-chamber, dual-reflex box and 230w RMS each...but I can count on one hand the number of times I actually played that loud. For regular driving, the aim is to get the subs to integrate with the sound coming from the front end, and that usually means driving them with surprisingly little power, as all I want is for them to fill in the bottom octave or so - the rest of the "noise" came from the front of the car.
It's also interesting to know that low frequency sound is less damaging than high frequency sound. Listen to a 50Hz tone at 110dB, and you won't see what the fuss is about. Try the same at 12.5kz, and you'll have your hands clamped to your ears because it's physicaly painful.