More power for moderate listening levels?

@atmasphere 
Here's Stereophile's test of the Harbeth SLH5 plus.
https://www.stereophile.com/content/harbeth-super-hl5plus-loudspeaker-measurements
It drops to 6 ohms at one point, and just below 6 ohms at another point.  But for much of the range it is over 8, and for some of the range significantly over 8.  With the degree of variation of impedance that one can see in the plot, how does one assign a nominal single value in ohms to describe the impedance of this speaker?

how does one assign a nominal single value in ohms to describe the impedance of this speaker?

Everyone does it different I imagine. 'Nominally' this looks like an 8 ohm speaker- if so that throws off my efficiency calculation by about 1.5dB. The impedance curve looks fairly benign, which is good. Its really the efficiency/sensitivity (which since its nominally 8 ohms are about the same thing) which is the issue here. Its assumed from the plot that the amplifier won't make a lot of power driving the impedance peaks- since the amp is a voltage source and the speaker is intended for that. 

My Klipsch KLF 20 with 100Watts Denon receiver sounds better, more live and dynamic with electronic and dance music than my Gershman Grand Avant Garde powered by 125Watts Plinius SA 102. As someone suggested, a separate system for dance music may be a cheaper solution if you have a space. Used KLF 20 and 100Watts receiver (or int amp) will cost $1000 or less.
By the way, my Harbeth C7ES sounds good at moderate listening level, but when cranked up with 125 Watts class A amp, it sounds stretched and nosisy. 

@atmasphere 
@twoleftears 

I purchased the Pass based on the measurements given by stereophile as well as reports of folks achieving great sound with lower power tube amps driving the Shl5. So far so good. With the volume halfway, the sound at my chair is fantastic with no strain. I'm a big fan of the sound when these two components are put together. The limitations of the system become apparent at much higher levels and with certain types of music. My assumption is that the bigger Pass integrated would clean up the sound at higher decibels and would be a measure of protection against speaker damage if one of my family members decided to crank it when I'm not around. Aside from the balanced design of the bigger Pass, does the extra headroom power-wise afford better sound at the levels that I normally listen to? For that matter, what are your opinions on balanced vs. single-ended designs? I would also be able to use the balanced outputs on my DAC.

As per Stereophile: XA25 80 watts into 8 ohms; XA30.8 130 watts into 8 ohms; and you can go up from there.