It's really tough to understand what people mean when they say "warm," because it's such a subjective term. But based on the mic you're currently working with -- a cheaper condenser with what I would imagine to be a harsher high end -- we can probably point you in a few directions that might be helpful.
The MK319, as John mentioned, is probably going to give you a much smoother treble response, based on what most of the users I talk to have been saying about it. Another budget mic that I particularly like that doesn't have a lot of that
cheap Chinese-assembled jacked-up treble response on speed sound to it is
the CAD M177.
If you've got the extra money to spare, I implore you to check out the Shure SM-7. It's a very mellow mic which still has plenty of presence to spare to help a voice stand out without ever being too strident. Probably one of the 10 best vocal mics ever made, if you ask me. You might also want to check out some ribbon mics by Beyerdynamic. Particularly something like the M-160, which can be had, used, in your price range. Ribbons are known, basically, for being about as mellow as you can get.
You might also think about some tube mics like Gidge suggests, but honestly, from my experience I don't consider anything by Marshall to be particularly "warm." It would be a lot like sticking a tube on your Sampson, which is really more like trying to cover up the problem rather than dealing with it head-on.
This is where I might say something like your mileage may vary, but I'm sick of that damn saying, so just use your own ears and good judgement and don't expect miracles.
