It depends what and how the splices were done. Splices on input coax lines shouldn't make much of a diff as long as the signal level is high enough for the components connected to it. Splicing of a component output cable might make a diff but it depends on the overall length and how the splice was done. The splice on an HDMI/DVI/VGA cable shouldn't make much of a diff since that's a digital signal and it either gets there or not.
The way to test for PQ change is to bypass all equipment (including the STB) and get an HD source directly - either via OTA and an ATSC tuner or direct cable connection and a QAM tuner.
The way to test for PQ change is to bypass all equipment (including the STB) and get an HD source directly - either via OTA and an ATSC tuner or direct cable connection and a QAM tuner.