What the player is doing. Try to do it. Something active, not passive. Good tone is active . 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.