It is willfully produced, it is not something that just happens because of the harp you use or the amplification equipment you play through. Tone is something the player puts in the note. But the timbre of different instruments, amps, and mics does vary. The degree or possibility of certain kinds of tonal manipulations can be different on different equipment and under different conditions. But remember, good tone is not passive, not just something that happens.
Good tone is control over notes that is done by the player.
Practice playing one note for a long time, listening to its sound, its voice, its nuances. Change it subtly, change it dramatically, play it soft, play it loud, play it gradually softer and softer, play it gradually louder and louder. Play it straight, play it with vibrato, change the depth of the vibrato, waver the pitch, oscillate the volume, let it fade out, make it end abruptly. Hold it for a long time. Work on higher key harps requires smaller movements and more subtle control of the muscles.