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Old 24th May 2019
Lives for gear

If the phantom power is clean it shouldn't do anything to the audio quality.
Preamps on the other hand can have self noise, typically high frequency hiss.
Low frequency may occur too but its typically a matter of AC EMF, Shielding, cable, or ground loops.

If your monitors are on when testing it can cause a feedback loop and given the fact the mic has a broad frequency response it could feed back in the low frequencies long before the highs. If the mic is on a stand, you can even have low frequency vibrations coming up through the stand which are being heard.
The easiest way to know is put on headphones and lift the stand off the floor and see if the response changes.

Given you say this happens when you crank the preamp all the way up, you can simply unplug the mic and see if you still have any background noise. This could be preamp noise and disconnecting the mic rules it out. I do suspect, as others said, you are dealing with ambient room noise. This is one reason room treatment to flatten the room response is so important.

What the real question is however, what is the background noise level with the mic set to normal operating levels. Cranking the preamp is going to reveal the flaws in your setup. Noise is cumulative in a signal chain. There may not be one source, there can be dozens of individual components which all add up to what is commonly referred to as the noise floor. The goal when purchasing or upgrading is to buy the best quality gear you can afford so self noise and distortion are minimized. That quest is like a Pyramid, the closer you get to the apex the more expensive beating back flaws becomes.

What you have to determine is if you can get by using gear that contains 10 cent resistors and $.50 op amps, or whether $10 hand made military precision resistors and $80 op amps make a big enough difference for you to spend what it costs to get the best. no man made electronic components are flawless, they have inherent flaws which are part of their design and the impurities added to make the components work properly are the same elements that cause noise.

The difference in high quality gear is first, great circuit designs. Second well built components using the best raw materials available.
Third and by far the most important is screening components. In mass produced gear, there is very little quality screening. Its cheaper for them to make a thousand boards and get a pass or fail with the board or components running under normal operating conditions.

High quality gear will have every component tested and only the best components cherry picked from a batch of parts. I used to do this all the time building and repairing gear. Back before amps had modular amplifying devices, we used to take a hand full of transistors and stick them in a tool called a load line analyzer. You'd pass different voltages through the transistors and you'd get different variances. If two transistors were off too much your stereo sound could wind up being unbalanced. You'd go through a batch to find the closest matches so your amps would push pull evenly or stereo channels had the same frequency responses.

You'd have different tests for most components and if you put the time into it you can get excellent fidelity from relatively inexpensive components.
Thing is its the manpower, not the components that make the gear expensive. A robotically board made in a batch of a thousand may only cost $10 to manufacture. The same circuit built by hand with all the testing to get the best possible results might cost a grand in comparison.

Again it comes down to how much noise there is when the mic is set to "Normal" operating conditions.
Most preamps will have the noise levels increase exponentially once pushed past 3/4 of max. If you're having to run it over 2/3 I'd say you likely have noise from poorly matched gear.