Ok low level advice
Ground is bad news, When you see something labeled "GROUND" start thinking what kind of ground is it.
Most times ground is actually 0 Volts, but 0 volts for "WHAT"?
In stead of saying
"0 volt audio reference"
"0 volt noise shield"
"0 volt signal reference"
"0 volt power reference"
"0 volt analog reference"
"0 volt digital reference"
People shorten that to just ground. Then seeing ground, think ground is ground and start connecting ground signals and in the process connect noise of one to the other.
So
XLR connector
https://en.wikipedia.org/wiki/XLR_connectorYou see Pin 1 is "Chassis ground (cable shield)"
Not really a part of audio as audio is on 2 & 3. This is protection from surrounding world.
Care should be taken on where and how this is connected to keep that protection high.
Balanced audio
https://en.wikipedia.org/wiki/Balanced_audioSo when you start connecting ground all over the place you get "Ground Loops" which are bad news.
You might not really see a problem until you go to extreme example of use.
Think of 1000 feet of cable connecting each of mike to preamp, preamp to down stream audio, preamp to power.
That 1000 feef of wire has resistance, capacitance & inductance. You also have a time delay.
If you shorten or lengthen the wire, all are still there, just the magnitude of vlaues changes and have a greater or lesser parentage effect on original.
So in an ideal world when you start with balanced audio input, you have a balanced audio output to speaker and every thing in between is balanced.
Fully balanced, you have one signal/wire going positive and other going negative. The current needed to make the change is on the two wires or signals. This costs more in electronics and sometimes wire so people try to be cheap and in process cause problems and you get less then you could have.
A lesser choice would be to have one wire changing and other fixed. Here you are still keeping current needed to change on these wires but still have a change on other end of wire.
So a smart person might first redraw that schematic such that you have different lines for each type of 0 volt use and put proper labels on them.
Here you have three.
Power supply 0 voltsl
Audio 0 volts
Other 0 volts.
You might see that instead of connecting XLR mike pin 1 on input side of preamp. it should be connected differently to prevent other world currents from effecting preamp as much.
Taking Care of return path could make a huge difference.
An example
If you know any thing about a switching power supply, you know that it is high noise. You can reduce this noise effecting rest of circuit by using just three connection points & good examples from manufacturer of switcher will show this.
So XLR mike pin 1 if connected to preamp circuit is connected in one spot only.
A good balanced audio amp has equal currents on positive & negative rails. When you can not have this, keep currents small and get balanced again locally. A good audio amp with RCA jacks for input will treat each as two separate wires(signals) even with one wire not changing voltage(current does change with signal).
The weaker the signals the more important this is.
So a good balanced amp is a thing of beauty and this beauty counters many problems. Think of power supply noise. The noise is positive going on one rail and negative going on other rail, and circuit only has to adjust/compensate to supply changes.
Now a balanced signal has a problem in that the signal is not always balanced and this can generate a DC offset. A PWM signal is one place this is used and the wanted effect. So a way to restore DC offset is needed.
So if you put this all together for preamp,
You want a separate floating power source of 30 volts with a local generated center tap. To reduce noise this power source only supplies the preamp
The preamp should not be effected if power is balanced at -+15, +-16 or +-14 volts. Power supply voltage changes should have little to no effect.
If you have to think ground then that poor audio RCA input jack shield is the ground. But notice that a good audio amp will have a ground stud to connect grounds so that RCA shield is back to being more audio 0 volts.