http://https//www.stereophile.com/content/kef-reference-5-loudspeaker-measurements
Looking at the impedance of your KEFs in the Stereophile review they're not exactly an easy load and you should drive them from the 4 Ohm tap of your ARC amp.
That impedance peak around 2.5kHz might also make your tube amp to highlight a little the frequencies in that region, making the speakers sound harsh.
I would try a SS amp that wouldn't have a problem driving them properly.
Looking at the impedance of your KEFs in the Stereophile review they're not exactly an easy load and you should drive them from the 4 Ohm tap of your ARC amp.
That impedance peak around 2.5kHz might also make your tube amp to highlight a little the frequencies in that region, making the speakers sound harsh.
I would try a SS amp that wouldn't have a problem driving them properly.