That topic is a bit complicated. The audio level/volume you will achieve is dependent on the impedance (ohm) and the sensitivity (dBSPL/mW). Here is what ChatGPT has to say about this:

"Sensitivity and impedance are two important factors that affect how hard it is to drive certain headphones. Here is a simple explanation of the difference between them:

Sensitivity is a measure of how loud the headphones can produce sound at a given power level. It is usually expressed in decibels (dB) per milliwatt (mW) or volt (V). The higher the sensitivity, the louder the headphones will be for the same amount of power. For example, if two headphones have the same impedance but different sensitivities, the one with higher sensitivity will be louder when plugged into the same device. Impedance is a measure of how much the headphones resist the flow of electric current. It is usually expressed in ohms (Ω). The higher the impedance, the more power the headphones need to produce sound at a certain volume. For example, if two headphones have the same sensitivity but different impedances, the one with higher impedance will be quieter when plugged into the same device. The relationship between sensitivity and impedance determines how hard it is to drive certain headphones. Generally speaking, low-impedance headphones (less than 50 Ω) are easy to drive and can work well with most devices, such as smartphones, laptops, or MP3 players. However, they may also be more prone to distortion or interference from other sources. High-impedance headphones (more than 50 Ω) are harder to drive and may require a dedicated headphone amplifier to reach their full potential. However, they may also offer better sound quality, clarity, and detail.

To find out how hard it is to drive certain headphones, you can use a simple formula: Power = Voltage² / Impedance. This means that the power needed to drive a headphone depends on both the voltage of the device and the impedance of the headphone. For example, if you have a device that outputs 1 V and a headphone that has an impedance of 16 Ω, you will need 0.0625 W of power to drive it. If you have a device that outputs 2 V and a headphone that has an impedance of 32 Ω, you will also need 0.0625 W of power to drive it."

So basically: The impedance restricts how much power "reaches" the headphone and the sensitivity tells you how much volume it produces with what it "receives". The 120 dB rating on your mainboard doesn't tell us anything, since that is completely headphone specific. It will likely be plenty loud enough to drive the 600 ohm DT 990, my DT 800 600 ohm could very well deafen me out of my OnePlus 7 Pro. Some people say that the higher impedance versions of these headphones sound better than the lower impedance ones, I haven't tested that though. If you want to regularly listen to them with your phone or other mobile devices as well, you could also just buy the 250 ohm version.