Re: 4 channels bridged to 2, midbass making a 'thumping' sound.....clipping???
Check that they are crossed over properly. High passed at around 80Hz is a good place to start. I would not go any lower than that, especially with that much power. They should be able to play quite loud without bottoming out.
I'm not too familiar with that amp but make sure the crossovers are enabled and set properly.
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