CyberSky, that's a very interesting question and there has to be a specific answer for a particular amp. There have to be scientific ways it could be measured but certainly beyond anything I have! Only have what's left of my ears
Whilst the actual amp itself can be measured, there are external factors to be considered (also can be measured), input (guitar's response) and output (speaker response) If you check with speaker manufacturers some provide graphs for each model. The only input graphs I have seen have been for mics (in fact EV used to supply an actual trace of the mic you bought, nice touch I thought) Guitar speakers I believe begin to roll off at the upper end around 5.5kHz whereas full range drivers will exceed 20kHz
As for the amp itself there are lots of factors, the OT will have a bearing as frequency in will effect what is put out depending on the OT's response curve and impedance match.
Other factors within the amp are the various filters that are formed and probably most notably are the pre-amp cathode bypass caps (values from 2.2uF - 0.68uF will show marked 'bass' reduction) Couping caps between stages and tonestack caps/resistors can be tailored to effect frequencies.
Power supply too effects how the amp responds as bass notes require more juice and when they don't get it compression occurs, often referred to as sag.
Perception in what we hear is another factor and tight or more defined bass makes an amp appear less bassy.