I recently purchased this unit, and while dollar-for-dollar I don’t think it can be beat in loudness (although quality is decent as well), there is no manual level for the subwoofer that is already set too high by default. I’m sure there are plenty of software solutions, but I’m not convinced that a hardware solution wouldn’t be easier and more reliable.
Would a bass-filter be better than a cheap EQ or neither? What say you audiophiles of L1Techs?
Honestly any hardware EQ I would feel comfortable recommending would cost a lot more than your speakers…
Anyway if you are set on analogue you will have to split from your PC into dual 6.3mm male to the EQ then from 6,3mm dual mono male to stereo 3,5mm female.
As for the hardware itself I would be looking at used 19" Graphical EQs just make sure they are stereo!
Parametric EQs would be a lot nicer especially for your usecase (Low shelf/cut) but they are also a lot more expensive.
In your price range I would be looking at used DBX or a Behringer gear. But with all the cables you would probably still be well above the speaker price.
Hope it helps!
In my opinion software would be the way to go because of the signal degradation of all the adapters alone.
If you are interested in Software just tell me your os and how much processing power you have to spare - or how audiophile you want to get 
Ps: don’t use guitar pedals they are usually mono and more importantly High-Z so it will 100% sound worse