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