My guess is that it is related to speaker cables.
It is unclear on the exact model and length that you are using. But I would suggest shorter cables.
Example if you are using the aural symphonic purples these will likely have high capacitance due to the twisted and braided design. This could cause the amp to be bright.
Also, in the Atmasphere manual it states that speaker cables can have a "rather dramatic effect"
FWIW.
It is unclear on the exact model and length that you are using. But I would suggest shorter cables.
Example if you are using the aural symphonic purples these will likely have high capacitance due to the twisted and braided design. This could cause the amp to be bright.
Also, in the Atmasphere manual it states that speaker cables can have a "rather dramatic effect"
FWIW.