General mountain dulcimer or music discussions
Marg, when I think about "tone quality", I think of three things pitch, volume and timber. String choice has an effect on all of these. First, let's define what I mean:
Pitch: the note to which the string is tuned. You also see this referred to as tension. More tension = higher pitch, less tension = lower pitch. Lighter weight strings can be tuned to a higer pitch than heavier strings because they require less tension to reach that pitch.
Volume: loudness. Heavier strings can produce greater volume than lighter ones
Timber: (pronounced tam-ber) This is all of those sound qualities that are not Pitch or Volume. Highly subjective and influenced by many many factors such as Wout mentions above: body design, wood choice, string materials, etc.
So, string choice is really a personal choice as to what you think sounds best on your instrument. For example, for many years I have played medium weight, John Pearse phosphor bronze wound strings on my guitar because I get greater bass response and a warm tone. Right now I like the McSpadden flat wound (squeakless) strings on one of my instruments because thery sound mellow and sweet on it and I have another strung up with nickel wound tenor banjo strings because they are brassy and bright. (and I found them at my local music shop in a pinch)
Robin Thompson plays an instrument with all plain steel strings of the same guage tuned to the same note.
Have fun and don't be afraid to try something new!