yamaha m-2 amp c-4 preamp set up

I recently hooked up a new pair of kef xq40 speakers to a vintage yamaha m2 amp (240 wpc) and matched c4 preamp. Bass and mids out of this world clarity. Truly remarkable. The high end clarity is razor sharp... however with this razor sharpness I am hearing some raspiness... and suggestions ... thanks.
Pjc