What you are looking for is actually a low pass filter. Low frequencies should pass it, high frequencies should be attenuated.
Audio Hijack is a high-quality macOS audio tool, which offers not only audio manipulation via an equalizer, but also a dedicated low pass filter.
Start Audio Hijack, add a New Session and select the preset Sweeten. This is a pre-build audio enhancement workflow for all sounds that are played by your selected output device. Then select the category Build-in effects and drag the Low Pass Filter block into the audio processing chain. It should look like the screenshot i attached. You can easily customize the cutoff frequency with a slider.
As soon as you press the start button on the bottom left, the audio effects will be applied.
In case a plain old equalizer is sufficient for you, you can try the free eqMac App.