I learned to trust my feelings over the years not the numbers..
false dichotomy - assuming you're human the results of perceptual psychology apply to you as well - "the numbers" for level matching, frequency response are necessary to establish a level playing field for effective listening for circuit, component differences
one of the lessons from Science based psychoacoustic testing is that even 0.1 dB, ~1% SPL is heard as a "sound quality" difference - and not as "oh that's just because one's louder" - few can tell the difference is solely loudness until the difference is >1dB ~10%
so to compare fairly by ear you have to match levels, and use some Blinding method to make sure the only information your brain can use is the sound entering your ears
at frequency extremes the level match requirement gets much looser: http://home.provide.net/~djcarlst/abx_crit.htm
the numbers in the abx threshold graph are useful to put upper bounds on "how bad" the frequency response can be without "sounding different" for the "uninteresting" reason that the level/EQ is off
the required level matching does require measurement, "doing some numbers" - almost any soundcard or PC with analog line level in is adequate for this if you use the same channel for all measurements
at the low end it appears that even 30-40 Hz 3 dB corner will not be "obvious" - but if concerns for subtle distortion drive you then as I mentioned above aiming for 5-20x lower corner frequency is a technical fix
the above doesn't have anything to do with claims that something "must" sound good just because the numbers are good - attending to these numbers are required for valid results from listening - whether you are gifted amateur, Golden Ears, Professional Musician, Sound Engineer...
comparing magnitude, orders of distortion to guess relative audibility goes more like this:
this doesn't address the (in)audibility of subtle capacitor dielectric distortion in a simple tube amp with 10000x higher 2nd order distortion from the tube, "simple" circuit's characteristics
Edited by jcx - 5/27/12 at 2:53pm