Here's an excerpt from Stereohile regarding HL5+'s measurements ...
The Harbeth is specified as having a sensitivity of 86dB/W/m, and my estimate of its voltage sensitivity was the same: 86dB(B)/2.83V/m. Though this is 1dB or so below average, I commend Harbeth for not inflating this specification. Although the Super HL5plus has a specified impedance of 6 ohms, my measurement (fig.1) indicates that the speaker's impedance remains above 8 ohms for almost all of the audioband, and that the electrical phase angle remains relatively small. The HL5plus will therefore be an easy load for the partnering amplifier to drive, and a good match for tubed designs.