If your arm has a standard DIN plug I would rent a range of cables from The Cable Company (1-800-fat-wyre). They carry a whole range of phono cables and by trying them you can deterine whether they are an improvement or not before you lay out the cash.(Because all rental charges go toward purchase you have nothing to lose...) I also agree with albertporter - youare dealing with such a small signal, seemingly minor changes can make a big difference.
