What ever you do, do not put anything between the cartridge and the phono stage!!! If your phono stage is being overloaded, then there is something either wrong with the phono stage, or you need to get a better one that has plenty of headroom for a good MM cartridge.
Avantgarde horns are very sensitive to partnering equipment and cabling, although I don't think this is the problem you are experiencing, as every source would be sounding the same...
You should never introduce any resistance, either in series or in parallel, between the cartridge and the phono stage.