Loud Hum From Monitors When KPA is Off

My Yamaha HS80's are giving off a loud hum when my KPA Powerhead is off. I've got my KPA Powerhead hooked up to my powered monitors via 14 gauge speaker cables. My understanding is that I should use speaker cables and not regular 1/4" instrument cables. The hum goes away when the KPA is on, but I'm not sure what's up here. Do I need to "deactivate" the powerhead part of the KPA and just rely on the monitors built in power?

Thanks in advance for any help!

Quote from paults

If the HS80s are powered cabinets , you should connect the KPA to them from the Main Outputs with XLR or Shielded cables or Monitor Output with shielded cables.

If they are unpowered cabinets, use the KPA powered output with unshielded speaker cable.

Yes, the monitors are powered. So you're saying I should use regular shielded instrument cable (guitar to amp cable) and not speaker cable (amp to guitar cabinet)?