2002 A3 Speakers - Replacement options

Hi Mike
You won't get any significant SQ benefits from replacing just the speakers.
The really weak spot on the A3 is the sub, so if you want a big increase in quality I'd start there (either with one of my stealth subs or traditional box in the boot).
The OEM sub is a plastic box in the left cubbyhole in the boot and even with the Bose upgrade you can't get clean punchy bass from what is basically a cheap door speaker mounted in a biscuit tin.
The cabin speakers are again limite by their size and more importantly the fact that they're mounted on a hardboard doorcard loosely clipped to the door frame. So even with £400 components up front you're severely restricted in how much bass you're gonna get out of them unless you go for custom door builds.
The front speakers are 4" components with a 1" tweeter, the rears are 6.5" coaxials in the 3 door, not sure about the 5 door.
The other problem is the power, the HU drives the fronts with a massive 2x 20w internal amp while the rears share a tiny amp located inside the sub enclosure. The Bose system is slightly more powerful using a 4 ch amp for the fronts & rears and a seperate 100w amp for the sub.
Both systems are woefully inadequate by todays standards especially in a £20k car (the system in the Golf IV is much better).
Upgrading the HU will improve the SQ but won't give you any more bass, and can introduce other problems like interference from the rears and popping when you turn the HU on.
Just by adding a sub with it's own amp you will dramatically improve both the bass and the SQ from the cabin speakers which then don't need to be driven so hard.

Yes it's the same size as the tupperware sub, so the CD changer still fits on top.
I can fit it if you can make it down to Southampton, or it can be a DIY job and I have a comprehensive manual detailing the instructions.
The enclosure is £90
The sub is £85
Alpine MRV-F400 amp is £90
Wiring - £35
Installation is £60
Or you can provide your own amp if you prefer, (needs to be 4 channel for non-Bose) with 200wrms+ available with 2 channels bridged to feed the sub (the other 2 channels feed the rears).
So £360 gets you a completely fitted & tested system or you can just get the enclosure and source everything else yourself.