Very interesting finding.
Although the SHL5+ is a little below average at 86db sensitivity, its impedance curve is quite forgiving, and really more an 8ohm than 6ohm speaker.
https://www.stereophile.com/content/harbeth-super-hl5plus-loudspeaker-measurements
BTW worth looking up the Stereophile measurements of the 30.2, 87db sensitivity and similarly undemanding impedance.
Although the SHL5+ is a little below average at 86db sensitivity, its impedance curve is quite forgiving, and really more an 8ohm than 6ohm speaker.
https://www.stereophile.com/content/harbeth-super-hl5plus-loudspeaker-measurements
BTW worth looking up the Stereophile measurements of the 30.2, 87db sensitivity and similarly undemanding impedance.