Thanks for the input guys, looks like I will be getting a case, I avoided using one up to this point because of the water cooling setup and difficulties fitting triple 120mm radiator. Cooler Master HAF 932 should fit my setup. Yesterday I tried unplugging few different components but it did not help, except that signals got a bit better when I unhooked the water pump, swiftech mpc355 18w, but it was hard to tell. I also happen to have an APC UPS with AVR (Automatic Voltage Regulator) functionality that supposedly boosts or trims incoming voltage. I poked around with ohmmeter and checked if there's continuity between motherboards screw holes and ground, and there was, so running a wire from PSU case to mobo's screw hole wouldn't help I guess.
I might try holding a grounded side panel from other case that I have or some sort of shielding to see if it helps. I'm using indoor antennas so just moving around disrupts the ATSC signal.