This is a tough question because there is no standard measurement or unit that describes warmth or brightness. If you are just trying to determine if an amp is bright or warm, listen to your headphones straight from the source with the amp bypassed and then listen with the amp inline and contrast the two sounds. With the amp, if the highs have recessed and there is prominance in the low end, then it would be safe to say that the amp is warm. If the effect is opposite, say the highs are more pronounced and the bass is reduced with the amp, then the amp would be bright.
Quote:
Originally Posted by
DavidMahler 
Is there a way to verify by ear whether your amp is bright or warm, without using a scope. Obviously your easy can sense brightness or warmth, but how can one determine to what degree is the amp's doing and to what degree is the headphone's doing, as well as the source. In order to determine brightness and warmth, there needs to be a fixed middle point. Can this be done with any accuracy without a scope?
If so, how?