You hook up a 1-2VRMS signal source (1kHz sine) to the input (across a small cap, 100pF ought to do) and measure distortion on the output (view on a FFT). If the second harmonic (2kHz) is more prominent than the third (3kHz), it isn't biased properly. You should be able to manage at least 0.1% THD at 1VRMS--that's harmonics at -60dB or less than the fundamental.
This is somewhat time consuming vs. setting the bias at a fixed DC (which will be close but not close enough), so if the factory wanted to cut corners that would be a likely procedure to skip. Or they may do it right, who knows? At least they use a trimpot, which shows they cared enough to make it possible to do right.
Here is a capacitor:
http://www.digikey.com/product-detail/en/C410C102J1G5TA7200/399-4448-1-ND/818305
You might say hey that looks like the cap that is already there. And it might be, you don't know. Ceramics come in many grades (they call them temperature coefficients), some have really low distortion (like C0G) and some are horrible (like X5R, etc.) Many modders assume all ceramic caps are bad--they are wrong, but if you don't know which kind you have it's not expensive to replace the ones in critical spots. I would measure distortion before and after, but I'm weird like that. Many modders simply replace and then always claim they hear an improvement.
With SMT parts it's really easy to tell: the C0Gs are gray, the X5Rs (X7R, etc.) are brown. I don't know why that is, but it holds across every brand I've tried. So if you have a gray one in the audio path, no worries. Also if you have a brown one that is a filter cap, no worries there either. Brown in signal path is probably bad.
Jon,
Thanks again this information is super helpful although a bit over my head.
Upon looking for the .22uf film capacitors I found many , many options, here:
http://www.digikey.com/product-search/en?vendor=0&keywords=.22uf+Metal+Film+Panasonic+I found this one
http://www.digikey.com/product-detail/en/ECQ-V1H224JL/P4667-ND/144461Which the novice in me seem to fit the bill, as it's for 50VDC
which I thought was closest to the 48vdc phantom power... is there a better one?
Also without an oscilloscope is there an easy way to generate 1-2VRMS signal source (1kHz sine) to the input?
Though I think I'm lost after that, should I even attempt to adjust the trimpot?
I've got a nice Tektronix DMM 252 Multimeter, but it's not that fancy.
I just received the mics yesterday and have not yet taken one apart to see and confirm that they are the same as the MXL603's.
which I will do soon and try to post pics too.
any further information is , of course helpful.
thanks
--Ian