"Amplitude Modulation" reception is generally very poor due to the nature of how it works, trying to pull in a crisper signal, usually ends up with the adverse effect, ie, as the station signal is increased, your also increasing the spurious signals either side of the one that you want!
AM signals have also suffered more recently, due to the massive increase in new uses for Radio equipment...mobile phones, microwave ovens, PMR, the list is endless really
You may be able to buy a filter that can be spliced in to the aerial coax (try Maplins) though I think you maybe flogging a dead horse!
Thom
