Look at the Stereophile review of the Kef Reference 5 (2017). Go to measurements and look at first graph, the impedance curve is the solid line. 1k - 5k they appear easy to drive. I would try reducing the 4' distance from speaker to side wall, if there are walls, to 3' for starters. How far to the back wall ? Hope this helps, Best.
How can I tell if I am overdriving the amp or the speakers?
I have a Hegel H390 driving KEF Reference 5 speakers and when I play something loud, the upper midrange ( saxophone, electric guitar, piano ) start sounding sharp and annoying. The amp is 250 x 2 into 8 ohms, stable down to 2 ohms and the speakers are 90 dbs, 8 ohm, ( min 3.2 ) 50-400 watts. I’m pretty sure it’s not room acoustics, but.
Thanks.
JD