There were three types of ICE installed in the Disco 2, depending on version; the lo-line which was just a head unit with a tape player, the Hi-line which used the Alpine R990 headunit with a six CD auto-changer under the driver's seat and a power amplifier under the passenger's seat or the Harman Kardon system which also incorporated a sat-nav.
The 2004 Landmark, the same version as my own was fitted with the Hi-line ICE system, not the HK as standard.
That is a known difficulty with both the Hi-line and HK systems, the use of an external power amplifier. Unless the replacement head unit has a left and right low level (non-amplified) outputs then connecting it straight to the power amplifier will over-drive the amp causing severe distortion. One way I can think of to correct the problem would be to introduce a pair (for two channel stereo) of custom built attenuators, but without knowing the various figures for outputs, impedances and required input levels, I couldn't possibly begin to design them.
You could of course wire some of the speakers; front and back, left and right without using the power amplifier.
There are quite a few threads on the forum about this problem.