hi all, just to get back to the initial question...
the factors involved in judging mic's regarding the tube vs fet issue can be so various.
my 2 cents, next to the things mentioned already:
amount and balance of low vs. higher order distortion
even order distortion (often referred to as "2nd harmonic") is dominant in both tube and early single ended FET designs. however, the ratio between 2nd and higher order harmonics can vary greatly within both tube and and FET circuits, depending on tube / FET type & biasing, amount of negative feedback. tube designs usually have more relative level of 2nd harmonic that FET circuits, with higher order harmonics rising in level later as overall input signal level increases. for both designs, distortion rises gently with input level even well above the usually cited max. SPL @ THD of 0,5 or 1%.
thus, a) real world signals passed through these mic's can exhibit a richer structure of harmonics / intermodulation products, b) especially in tube designs, transients may exceed the max. SPL by a considerable amount without sounding grossly distorted. Early FET's tend to sound less forgiving there, since at that point, the amount of higher order harmonics is already greater, increasing the risk that the resulting distortion is actually being percieved as unpleasant.
there are exceptions such as the schoeps circuit with one FET followed by an active balanced BJT output stage. here, you get dominant odd order distortion (3rd harmonic), but with less relative level than the above designs.
modern FET mic`s mostly include several active stages with high amounts of negative feedback, resulting in high headroom but also lack significant distortion of any order within their dynamic range, until just above the max. SPL point, hard clipping occurs... so these mic's amps sound "clean" within their operating range but will reveal gross distortion even when slightly overdriven.
the transformers of tube mics can vary greatly in design - some metal alloys can exhibit high distortion (dominant 3rd harmonic) especially at low (!) levels, so depending on the materials used, low level signals may also sound different regarding harmonic /IM structure. besides, some metal alloys tend to rise gently in distortion as input is increased, others retain low distortion up to a certain point, beyond which they will rise rather steep (compareable to soft clipping). Some designs will exhibit considerable rise of low freq. dist. at high levels, others only a marginal rise thereof.
not to mention possible interactions between freq-dependent and non-dependent resistances of mic output and mic-pre input (properties of maybe present transformers, varying overall input impedances...)
so IMO it's not all that easy =o)