I won't try to add anything to the technical discussion about big stereos, but let me say as a past owner of several large 4X4's that investing in some decent sound proofing will pay dividends, not just to the sound of your stereo but also general driving pleasure.
There are three types of sound you want to suppress, resonance (vibration), wind and transmitted. Resonance is treated by the use of damping pads that stick to large panels (roof, doors, floor pan etc) to stop them vibrating. These pads are quite small, generally 8" x 10". Transmitted sound is reduced by the use of sound deadening mats (Dynamat, Acoustimat etc) and is fitted over any resonance pads you fit. Sound deadening mats can be fitted in the engine bay to reduce engine noise in the cabin.
Wind noise is much harder to suppress, it is caused by air being sucked out of the cabin past window and door seals. It will be hard to do anything about it on a 10 yo Disco. You can get new door seals, make sure the hinges and latches align to get a good seal. 'Putty' up holes in the body and thats about it.
Acoustikit used to do kits for sound proofing cars, but it now appears they have gone bust.
