Do you have a USB dongle you use with your phone to plug headphones in? You might test that one first. These headphones do not seem power-hungry, so a dongle might be enough. And surprisingly one of the most cost-effective options is the Apple dongle for 10 dollars. Generally tho, monitors are terrible sources to plug your headphones in. Most of the time, the output is more of a marketing gimmick, not a well-engineered device.

If it sounds bad, Just get a cheap Amp/DAC. Something like this should be quite satisfactory. And if you don't want to dive into the rabbit hole of software paramnetric EQ, you have basic tone controls by physical knobs there.