@sleepy.weather
It kind of strange for Antilope not providing the full technical specifications (input and output impedance) of their product, given the price they are charging for it.
First, make sure you use balanced TRS cables to connect monitor outputs and line outputs to the Topping L50 balanced TRS inputs.
Given the fact that you, probably, already use the monitor outputs to send the audio signal to your monitors, I would check the monitor output trim level setting first, because you may have it calibrated to a lower level then the line output trim level.
You have two options to choose for: +14 dBU or +20 dBU.
If you mix în K14 system, you may have it set on +14 dBU, and that's why your monitor output level is lower than the line out level. The default level setting for line output and monitor output is +20 dBU.
You should also check your equalisation settings on the monitor outputs, because you may have it set to compensate for your monitors frequency response.
Finally, I would not rule out an issue with the Antilope interface drivers, also. Therefore, a reboot of the unit, especially after a firmware or driver update would be advisable.