Thanks, what a useful website.
Looks like it'll cost me the best part of two grand. I need FM immunity at some point and the cheapest way is for a filter for my KNS80 Nav 1 box which is still about £1600 alone. Maybe I would be better ripping it out and installing a GNS430 to kill two birds.
As you say the map on the KLN94 is not fantastic, and doubt it justifies £2k just to replace the KLN89B.