Hey Mike,
for M49B most important are R3, R6, R7, R10 (also R4 could be carbon, i forgot about it).
For the high impedance resistors take whatever you will find. Rather there's really hard to find original type of resistors in this range of resistance.
R8 and R9 is much easier to use 1% metal film resistors to get proper divider for polarisation voltage matching.
You schouldn't notice dfference in sound in this position. If you will find 1M from old resistors, for example 1%, 2%, 5% then you can buy more of them and measure it, but it shouldn't change anything. Defintely there wouldn't be degradation in sound if you will use old resistors.
All mentioned "improvements" i think are not so improvements at all
In M49 from B version Neumann add so called "broadacast filters".
It's the part of the sound which most people call M49 type of sound.
It's that "mojo" of M49.
Not without a reason you will find many posts about less low end than in other designs and that it fit female vocals the best etc.
C4 is a feedback capacitor which have really good properties. Removing it you will get little bit different sensitivity as also response. It makes more linear response. Definately i would keep it.
C2,C3,R3,R4 forms another feedback but with hipass filter. Similar you will find in u87.
This is exact, most popular, M49 sound.
I build C version with M7 capsule and tried with filter and without it. In both cases it sounds good but really different. I tested it with cheap k47 and to be honest it sounds better with filtering.
I'll be rebuilding this microphone with different k47 (M7 go to different mike) and definately it will be using filter.
I would definately keep original schematic here.
For more low end you will have to remove all these parts except C2 which you should connect direct to ground.
Since it's feedback you will also get a little bit different sensitivity.
Better option for more low end is just higher capacitance of C3 and all other things except low end roll off stay the same.
C6 is LPF. I didn't noticed any difference for both capsules in high frequency area. It can add some little distortion.
If there wouldn't be C4, at C6, 200pF will make more hi freq roll off than 600pF in original design.
It also filtering RF.
Definately you should consider use of S2 switch - this is part where you notice improvement, and Neumann not without a reason add it. If you have no option to use it, then try C1 470pF. For all other parts i would keep original type of circuit.
I newer understood that type of modifications which people like Oliver Archut and many others liked to introduce.
Mentioned distortions aren't that kind which you wouldn't like if you will hear it at all.
So what? To looks better on a paper from electronic design side?
If distortion or noise are not introduced by electronic parts degradation (due to time of use) then it's not worth to change good design.
I agree that some DDR microphones need some little improvements, but also not so drastic like you will find propositions from the "specialists".
Good microphones doesn't need castration of their unique features.
What for making all microphones similar to each other, with everything hyped etc.
What for making diy microphones based on clasic designs?
Instead you could buy few Rode K2, it's tube microphone which is designed very well from electronic designing point of view.
Less distortion, high sensitivity, ultra hyped filtration and response and other blah, blah, blah.
But there's one problem
It's sounds ugly
It's just like you would buy old neve console and remove all the preamps to fit to the enclosure modern "hi-end" opamps and make it transformerless or use lundahls
