ok, for 800 we could say
HU - £250
fronts - £200
Sub - £150 (go 2nd hand i think for a decent 12" sub)
£200 on a decent 2nd hand 4 channel amp, or get 2 lower end 2 channel amps
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Thats what i would do, but the chances are you will spend more on it though time, so i wuold make sure i bought things that could be upgraded easyily, eg, a 4 channel amp to run fronts and sub, then buy a 2 channel amp to run the sub and run the front components activly
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ok - now i havnt incorparted wiring kit, that shoudnt be the earth, and sound deadening, should do at least your doors, could set you back anoter 100 notes
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Also DO NOT GET 6x9s they a horrible things (well 90& of them are) they arent usually ver good quality, if in the parcel shelf, interfere with the sub (sharing enclosures) and the 6*9s will get broken if your sub can more some air - which any decent sub will do - and the 6*9s will probably be slightly out of phase, causing bass cancelation issues.
I would go for on a budget like this along the links of
HU - a lower end alpine unit, or a new JVC, or a decent 2nd hand alpine
Fronts - DLS - say a6 components, not sure how much they go for, or crystal
Sub - ive just bought a crystal comp12, good sq and is loud, but might be a bit out of range, how about 2 comp8s - you be surprised how loud they are!! ive got 2 * 15w6v1 (JL) in large ported boxes in my project car, i would say a 15 may be hard to power properly with your budget.
Amp - old skool rockford from US ebay, get a 125.2 for £125, give about 600rms (not to be mistakend with max power that sony and that like quote)
If you give me more details, like, is 800 top budget, kind of music, how much boot space your willing to sacfice il be able to help more
PS - sorry for the long rambeling post, hope you can sort through it to get some info out of it!!
Rorrie