That is some solid advice if you ask me, and on two accounts; first -stay away from that Behringer in bridged mode, it likely killed your previous speakers due to clipping and is likley to do so again (the power rating is inflated through the roof IMO) and secondly; look for high efficiency speakers with more surface area if you need more SPL.Inferno, You seem quite happy playing with your synth,,, so why not approach your end-goal in steps? Work on a mono set-up for the synth for now,,,, get your amp fixed, install a quieter fan.and stay away from 2-ohm operation. If you did that, your new fan would probably rarely need to kick in. There's no point in going stereo on the synth. You'll reduce costs for now, and you can add on later. And,,,, stay away from running that Behringer in bridge mode. Get a more efficient speaker if you need the volume.
P.S. I admire your tenacity.