I found this place when I was thinking of doing something like this when my DSP AMP broke.
http://www.uniqueintegrations.co.uk/services/conversions/range-rover/ Here is the quote they gave me:
This was a quote to change the stereo to an Alpine 178bt which will give you Ipod control via USB and Bluetooth hands free and audio plus a single slot cd player. We would then fit a pair of JL Audio C2 650 front components and run them of a JL Audio 4 channel amplifier running them and the factory subwoofers
Alpine 178bt
JL Audio c2 650 speakers
JL Audio 4 channel Amplifier
2 x RCA cables
Amp power kit
5 metres Speaker cable
Steering Control Interface
Radio Facia Panel
6 hours labour(60+vat an hour)
Total £1234.99inc vat
This system was using the same head unit but we would change the front speakers to a pair of JL audio TR speakers which is their entry level speakers and not run the subwoofer.
Alpine 178bt
JL Audio TR 650 speakers
5 metres Speaker cable
Steering Control Interface
Radio Facia Panel
3 hours labour£60+vat ban hour
Total £680.98inc vat
More than I wanted to spend so I found a replacement DSP and fixed it that way.
Good luck.
Carl......