Did you try a different input cable? Or, another pre-out from the Preamp? I would try the center channel pre-out for a test to rule out the sub out from the preamp. if another cable and different pre-out produce the same loud hum, then you should contact Outlaw for assistance.
Main HT <> 976 <> 7200 <> Ultra-X12 <> sapphire ST2 mkII RF/LF <> sapphire SC center
sapphire ST2 mkII SBR/SBL <> sapphire SS SR/SL <> Verizon/DVR <> Vizio E70U-D3 <> Oppo UDP-203 <> Amazon 4k Fire TV