We use R-122v tube version which was the top of the line before but now they have FET version. I do not know what sound different between tube and FET, one thing we can sure is a hand made silver mic cable is much better than the cable come with, the pre amp is my design step up silver transformer in input drive WE437A coupling to WE300B SE 600 ohm output to Studer A80's record amp, this is the way we are in recording
Silver shouldn't matter but the tube outputs are pretty high-impedance and thus more load-sensitive so a cable with lower capacitance can help. I used special coax in the past to get the capacitance down in the 10 pF/foot (~30 pF/m) range, down from typical cables' 20 to 30 pF/ft (60 ~ 90 pF/m). And short cables... Ditto the transformer, changes in core material and winding probably swamps wire contribution, but a custom design could certainly be better than stock. A lot of stock transformers include small capacitors for RFI suppression and those can affect frequency response.
JFET's have a tube-like transfer function so should sound very similar. May have lower noise but probably overload sooner (not normally an issue for a microphone, I have seen that mostly with turntables).
We have not measure but we do tested, set up mic and listen in far away room, one guy talking and walking round and round two mics others listen by the monitor system, we found the home made one has more emotion and more live also good sound stage too , I guess those differences are more important in recording than frequency response
Not too many people have tape machine and tape price a bite too much too so just a small numbers of people know our recording, that is why I am trying to push the LPCM 96K DVD format which direct dubbed from our master tape and play back by blue ray player as transport to DAC, will have quality very close like analogue in digital format