I find that it's not really the strings but the dulcimer itself that either does, or doesn't give you the warm mellow sound, the type of wood also plays into it in a big way, such as comparing Cherry to Walnut or red cedar. tuning is the next big one, for me personally anyway, I find the CGC tuning to be very warm mellow sound, but have also found it doesn't always "work" on all dulcimers, it really depends on the personality and voice of each individual dulcimer.
I just picked up a couple Warren Mays, and find that they overall have a very mellow voice.
I'll be posting a new forum at some point highlighting the Warren Mays, I got a lot of great feedback from folks here at FOTMD about Warrens dulcimers, and I'm not disappointed in the least, some of my favorites to play, and they handle the CGC tuning like a champ!!!