Oh, I'm sorry i meant hisses as per sibilates, there was no background noise, just more pronunced ssss :).
The reason for the sibilance is... unknown :) I mean why it's more pronounced with certain amps. We know most amps have a basically flat frequency response and totally flat in the "sibilance range" (which i believe is around 4-6khz), so this remains a mystery :)

























