Treble harshness - interconnects as culprits?

Hi,

I have good quality speakers, highest quality integrated amp,a high quality transparent/ very responsive dac attached to a sonos (via coax) which connects to my music library on a nas.

The system is very transparent and overall great ... Except:

I have recently observed some sibilance and high frequency/ treble harshness on mainly live female vocal recordings like 1960 Edith Piaf, but also on more recent recordings. Even at very low amp volume levels.

The only components of poor quality are my speaker cables and my radioshack analog RCA interconnects between dac and amp (only RCA input). Could the interconnect cause this? Or do I have bigger system compatibility issues?

Thanks!!