ISOL8 is a freeware mix monitoring tool that is designed for monitoring different frequency regions. It is released by TBProAudio and can be used in VST, VST3, AU, AAX, and RTAS plugin formats.
It also features a filter bypass switch, monitoring channel adjustments (solo buttons for the Left, Right, Mid, and Side channels), an output volume control, and an attenuation switch. It claims to more streamlined than a multi-band compressor as it allows precise monitoring of the mix.
Good news is that it is free to download and does not require a sign up from the developers website.
Read more about this here: https://bedroomproducersblog.com/2017/12/06/tbproaudio-isol8/