Speaker upgrade help

Hi Guys. I'm new to the forum and a first time Audi owner.
I've recently purchased a 2000 A4 1.8T Quattro Sport and love it. I'm quite happy with the factory fitted sound system and since finding the hidden equaliser on the symphony headunit i've got it sounding better but i'm just wondering if it's worth upgrading the speaker system running off the head unit to get better sound and bass. It has a 4 speaker setup I think consisting of Front components and two in the parcel shelf. Haven't checked the rear doors but don't think it has any. I've noticed that the low Bass causes a bit of distortion on the factory speakers and was hoping to eliminate this with an upgrade and also get a better clearer sound from the components. Can anyone suggest a good working setup and provide me with some tried and tested speakers i could consider for the front and rear. I don't really want to remove the symphony system as I as I want to keep it as standard as possible and not bothered about amps and subs, just want a better sound with clearer bass.
Any help is greatly appreciated.

Thanks mate. Looks good. Do you have them running off the symphony or have you gone for an aftermarket one? I'm currently looking into MP3 options with the symphony but may just go with aftermarket head unit. Just worried about the console looking rubbish with a single din head unit in there! Do you know how hard it is to replace with an aftermarket head unit?