Bose setup

IMO you would be better of with aftermarket audio equipment

Its not that good anyway...

Yeah, Bose is pretty hopeless especially in the A3. And to retrofit one would be more complicated than installing an entire aftermarket system as all the wiring is different, it uses a 4ch amp, seperate sub & amp and all the speakers would have to be changed.
Ideally go aftermarket, but if you stick to the Concert then just by adding a sub you can get pretty good results far better than the Bose setup. The sub is the key as even with the Bose setup the OEM sub is housed in a plastic box which gives you that signature woolly/muddy bass that we all aspire to.
I do a custom stealth sub enclosure using a JL Audio sub which replaces the OEM unit and the transformation is pretty outstanding if I do say so myself.

How much is one of your custom subs etc to fit a b5 s4 with standard concert. How difficult is is to fit, I guess you need an amp ?. Got any pics of an install on a b5 ?

A3/S3 only I'm afraid, you can get stealth sub enclosures for the B5 from Auto Acoustics. The only reason I did the A3 was that the exisiting sub was abysmal (Bose & non Bose) and no one did an off the shelf unit.
B5 Sub

Thanks