Harp guitars are a unique instrument, aren't they? The six or seven bass strings on a standard harp guitar are typically tuned in the diatonic scale, giving the player a bass "drone" to accompany their playing. From there, many more possibilities can be incorporated, all to give the player more octaves to work with beyond a standard guitar's limitations.
Here's an excellent video by Muriel Anderson, explaining the versatility of her newest Michael Doolin harp guitar (seven bass strings):
And here she is with a lovely composition on her older guitar (six bass strings). The sound is much more full due to the bass strings, which really set a wonderful platform for Muriels' excellent harmonic technique:
The YouTube comments that struck my "funny bone" were about Mr. Bennett's resemblance to Albert Einstein. One person said he enjoyed Bennett's guitar playing, and also liked his Theory of General Relativity ... almost as an afterthought.
ACOUSTICS: Cordoba D10-CE / LaPatrie "Concert" / Takamine GD30CE-12 / Norman ST30
ELECTRICS: EP Les Paul Custom Pro / Gretsch Streamliner G2420T / EP Thunderbird Pro IV bass
AMPS: Peavey "TF" VK212 / Traynor AM150T / Fender Rumble 150
EFFECTS: Boss ME-80 Multi-effects / Ibanez WD7 wah / Grand Orbiter V3 Phaser