Problems with Harkte HA2500

[BassSkank]

I have a Harkte 2500 head. I'm running a Line 6 Bass Pod Pro through it and am using a GK Backline 4x10. When I press the in/out button next to the EQ to the in position I get awful feedback, buzzing and the volume goes incredibly high. I can't use the EQ anymore because of these problems, so my sound is not nearly as good. Any help as to what is causing this and if I can fix it?

[B-string]

Larry should be along (the companies founder and president) with some help (he even posts his cell number!).