Buy a used home cinema Subwoofer with speaker level inputs, sometimes called High level inputs.
Wire cables from the cabinet speaker terminals to these speaker level inputs (LEFT or RIGHT inputs) so it's in parallel. Turn on sub. Really improves low-end audio on newer Sterns.
You can wire 2 games to one sub, one to the left +/- inputs and one to the right +/- inputs.
I have 2 Tannoy HTS subs (£100 for both off eBay, almost new!) and 1 Mission M2Sas (from old home cinema set-up before kids arrived) serving 6 games.
Turn crossover freq to Max (150Hz ish) and then adjust the volume to suit.