The only things I can come up with ( off the top of my head is ).

If you have a ton of exposed pipes down there, cover them up with some insulation.

Next thing...while you might have the wires for your gear ran fine, you could be getting some issues with the electrical work running through your house.

To help trace these problem spots...plug a single mic into your mixer, put some headphones on...and "scan" the mic around the basement to find problems.

Beyond that...check all the stuff in your setup chain one by one, and experiment with placement. Also run power down there from a different breaker then you are now, just cause your grounded doesn't mean its a good ground.