Please check the estimated power budget below, which explain how much power and SPL(Sound Pressure Level) 5K can deliver.
As explained very well in the table below from SHURE, 120 dBSPL is threshold of pain, 110 dBSPL is Dance Club, 100 dBSPL is Factory.
However, the estimated Sound Pressure Levels are assumed and calculated with 0dBFS pure tone input, which is the maximum digital input as we can have. Thus, depending on your music source's headroom, the SPL can be lower than the estimation.
5K 3.5mm Single DAC can deliver the output power up to 80mWatt, while 2.5mm Dual DAC up to 240mWatt. For the low impedance unit, the current saturation occurs before reaching the maximum volume level. Please note that even in the current saturation, the DAC and HW are all safe, but you will get the output signal saturated and clipped.
In most cases, if you're using high sensitive IEMs and Earphones, you will never reach the saturation point before experiencing the threshold of pain. The below references are mostly for headphones with low sensitivity.
Audio Technica ATH-R70x
*Please note that DT990 PRO doesn't support any interface for balanced cable replacement.