Both the Atlas Magnum with KT120 or the earlier review with EL34 had High frequency problems as far as bench tests went. Maybe your hearing this.
" Though I must admit to some disappointment with the amplifier’s lack of high-frequency linearity, in this respect it is no worse than the original EL34-equipped Atlas"
Read more at https://www.stereophile.com/content/rogue-audio-titan-atlas-magnum-power-amplifier-measurements#huw4...
" I wonder why a touch more negative feedback was not used to make the high-frequency performance more linear"
Read more at https://www.stereophile.com/content/rogue-audio-atlas-power-amplifier-measurements#riKwozYLl0XETCR8....
Cheers George
" Though I must admit to some disappointment with the amplifier’s lack of high-frequency linearity, in this respect it is no worse than the original EL34-equipped Atlas"
Read more at https://www.stereophile.com/content/rogue-audio-titan-atlas-magnum-power-amplifier-measurements#huw4...
" I wonder why a touch more negative feedback was not used to make the high-frequency performance more linear"
Read more at https://www.stereophile.com/content/rogue-audio-atlas-power-amplifier-measurements#riKwozYLl0XETCR8....
Cheers George