A simple free or cheap plugin to make the low-end mono?

Then you already have it.
Load up Maximus on desired channel, and set it to default.
Down to the left you have a knob with 2 speakers pointing away from eachother - that's stereo separation. Set it all the way to the right to make it mono.
Each band have one of these, so just adjust the bassband until it's good, and sum everything to mono.
After that I would recommend you doing some compression on the bass as well, to even out the phase-issues you introduce when summing bass to mono.
 
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