Test results for the SM58:
The transformer that was found in the SM58, with a source impedance of 30 ohms circa, is flat from 20-20 KHz, with a slight dip towards the top.
The primary and secondary DC resistance of said transformer is very low, it is a 1:4 ratio. I suppose that, the low DC resistance is an indication of good power transfer?
Using 150/200 ohm 1:4 mic input transformers worked as well, into the soundcard, perhaps because of the 'easy' load. I imagine that in a transformer coupled preamp, with a step up, the original is better suited.
I also used a 60's UTC transformer, which had low impedance taps and this worked best, in terms of level, but the bass response was rolled off, and there appeared to be some interaction between the voice coil and the transformer, in terms of resonance on the top end.
So, considering that the transformer furnished with the SM58, is itself relatively flat, when presented with a constant source impedance, I am guessing that the voice coil is quite sensitive to load.
The mic sounds best, to me, with a high impedance, although for close pickup, there is an excess of bass, due to proximity effect.
Also noteworthy is the effect that the size of the cavity inside has on the mic's response in the bass. Filling the cavity with a solid or foam changes it's response and overall sound to a degree.