OK, I've just checked out the specs of your amps:
This is them, right?:
The specs are (RMS) 2 x 125watts or 1 x 380watts.
These are really good amps, and no replacement is required in my opinion.
I'd look to run one off of the front RCA pre-out to power your front components (see below).
I'd run the 2nd off of the non-fading pre-out powering a 10" sub in a box.
I personally would ditch the stealth shelf and replace it with a genuine item (your car sounds like a real minter, try and keep it as standard as poss) and get some drop in replacements in the standard rear location (parcel shelf supports). These will be head unit powered only, faded right down to offer some rear fill only.
You might find you don't even want rear fill - try having a listen with nothing connected first.
Your front speakers are currently your weakest link.
The 13cm door speakers have an rms rating of 35 watts and a sensitivity rating of 90db.
I'm not entirely sure how these are wired in terms of crossover, but it seems a little wierd to be running seperate tweeters alongside a 3-way speaker.
I would look to spend a chunk of money here - you're going to struggle to get a set of 5 1/4" components that will handle all of your 125wrms power for anything approaching sensible money, but £150 will get you a
vast improvement over what you have now.
Have a look at products from Infinity, DLS, Rainbow or Hertz.
As for a subwoofer, you're looking to approximately match rms watts but play close attention to box volume. A great sub in a box twice as big as it should be won't sound as good as a budget sub in a weel sized box. I'd advise going to your nearest car stereo place and having a look at sub and box matched packages. From a shop you're going to be paying circa £150 for anything half decent.
Alternatively I have a 10" JL sub in a small JL box for £100...
