you could get a stealth shelf. there made of mdf and the speakers bolt underneath so the shelf looks standard from the top, also helps give a better sound
you won't get a stealth shelf and 6 x 9's for £50
If you want bass you will need to save just a little more and go second hand. Quality sound systems cost a fortune from new. Your best bet is Ebay or look out for mates who are renewing their stuff.
I got a 4 channel amp from halfrauds for £25 in a stock clearence (R.R.P. £90
)
A mutant 12" sub in a ported box, nearly new from a mate who needed some quick cash for £20.
Wiring kit (had to pay for this but meh..) that was £20
Fitted myself using guides on this forum so addd another £0 (soon to be £10 when I
donate..well worth it but not required)
So thats £65 and I just got some 10cm speakers cos my others burst
which was another £15.
So second hand, and shop deals my system cost me £80. Buying the amp brand new alone would have cost more than that.
To be honest I wouldn't go the 6x9 route. Deffinatley sub+amp combo (these are hidden so less chance of a break in). You don't even have to buy it together, just save a bit of cash and buy a bit of your system, then save more and buy more, piece it together, that way your not braking the bank to buy it.
It can be done dead cheap, it won't be the best system in the world but its better quality than these £200 deals from motor-world / halfrauds.