First of all I would not pump any more money into this set-up.-which is being used as a work station and a defacto hi-fi set up
Your aim is to improve the sound of what you have at minimal cost.
I would replace speaker cable and inter connects though.
I would also lose the table , display and keyboard , electric fire , flip flops and any other clutter. Use headless control via mobile phone.
Get the speakers away from the walls.
The PC it looks ancient and may have a limited speed cpu ? - Replace HD with SSD if not already done.
A clean Install of windows server 2019 - Free trial available Reduce number of services running in back ground.
Remove as much clutter on SSD and install HQPlayer -free for 30 days. Employ highest HQP settings that the cpu will permit.
Pull CPU out and replace with quality thermal paste
Install ROON +HQPlayer
Ethernet connect from router to pc -disable wi-fi