Having just fitted an after market head into my Disco 1 ES, I can tell ya that the sub takes a line level feed from an dedicated output from the stock head, and a 12v+ from it too. I have piggy-backed the 12v+ feed from the post ignition switch feed so the sub power goes on and off with the ignition, and taken the audio line level feed from an aux out on the head. The amp in the rear door has a simple filter in it to try and keep low end only going to it so connecting it like this will work, but it isn't as good as the original. The stock head also filters and eq's the sub feed coming from it. I am going to fit a different aftermarket head in the near future that gives adjustment of the sub out so I can tune it better. All in all it took me an hour, and at the same time I added a USB power supply into the cavity in the console running via the switched power feed to power the phone/pda and an ipod holder, with the wires all buried and neat looking!
The main speaker connections are direct via the ISO connector and is directly interchangeable if your aftermarket head is 'standard' iso wiring.
Regarding the interference, the stock heads are actually very good, and well filtered (older and simpler IMHO!) so less reseptive to the interference. Nwere heads with 23956 different features are less filtered and more prone to rubbish, so running seperate line from battery will help. Be careful to only replace the constant feed, not the ignition feed.
WARNING
After the 1 series, landrover started going mad and having remote amps, and the heads just sent line feeds to the amps, so don't try it like this with a newer vehicle! To fit an aftermarket head you have to either find a head with enough line level outputs to run to the amps, or bypaww the amps altogether and use the head amps to power the speakers
HTH
D