S3 8L Think i ve blown my sub

Retail about £400 IIRC (total joke!). I'd do a bit of investigation first as it depends what the symptoms are as to whether it needs to be replaced. They do come up on ebay for around £70-£80, but it might be an easy fix. One common problem is that the screw holding the mid section of the enclosure breaks away leaving the front of the sub flapping in free air, which can make the sub sound even more dreadful than usual. With the unit playing try putting your hand down the front of it (where the two 4" drivers are) and push towards the front of the car, holding the front against thew tupperware enclosure. If this stops it farting then you just need to repair the broken plastic bit where the middle screw is (shown here):

If it a more serious farting problem then yes the likelihood is you've blown the actual drivers, so easiest and cheapest option is to replace the whole unit:
http://cgi.ebay.co.uk/NEW-GENUINE-A...arts_Vehicles_CarParts_SM?hash=item3ca5c17578
I'd double check any advertised subs on ebay as many are advertised as being Bose when in fact they are just the non Bose unit. The correct part no. is 8LO 035 381H and it should have a black plastic front with two 4" drivers. The standard non Bose sub has a metal front with a 6.5" driver. Or you could buy this one from an A6 and nick the 4" drivers out of it then sell it on as spares or repair:
http://cgi.ebay.co.uk/Audi-A6-1998-...ent_GPS_Car_Amplifiers_PP?hash=item1c0d15f9ac

Better still upgrade to a stealth sub:
http://www.dizzythedog.com/index.php?f=data_audi_a3_stealth_sub&a=0

The sub does come apart easily, but getting to it requires removal of the 3 cargo anchors, the boot lip trim then the boot side panel. The side panel will still be attached at the back with a single screw that requires removal of the rear speaker panel in order to get to, but you don't have to do this as you'll have enough room with the screw still in place.

That's good news, it maybe just a broken screwhole. To repair all you need is some Araldite, then when it's set don't overtighten the screw.

Araldite Rapid in a the tubes, don't buy the syringe as you can really only use it once

Don't think the tupperware enclosure is made of steel

Only if you can find 4" subs running at 1.5ohms

The impedance is the load presented to the amp. If the amp is rated for a 1.5ohm load it will only deliver its rated power when connected to a 1.5ohm load. If you connect a 4 ohm load then you'll get less than half the power from the amp.
Those 802 drivers would certainly be worth a try