I think usually when people are looking for a "warm" sound they are looking for an amplitude response that is tilted towards the low end or in other words the OP is trying to avoid a bright cartridge. Perhaps his system is already on the bright side and he is trying to melo it out. I have not heard the Nagaoka MP500, but my understanding is it neutral leaning towards bright. It is certainly a fine cartridge. Lew certainly likes it.
I just ordered a Soundsmith Hyperion MR. Got an amazing deal on it. The highlight of my childhood life was getting a new cartridge. Some kids never grow up:-)