"Leakage Current" is a NORMAL side effect of using EMI/RFI Filters on the Power Lines entering most Electrical Appliances in order to meet FCC Class B (in USA) Radiated Interference and Susceptibility Limits. Sometimes these filters are part of the power entry system and hence are ALWAYS connected, even when equipment is OFF. The large value Capacitors between "HOT" and "NEUTRAL" as well as from each to "Safety Ground" can have a high resistance leakage current that can result in AC voltage levels of a few volts....and when disconnected, can retain a DC voltage level across the Capacitors of nearly the same voltage until it leaks off. Large Appliances, such as Dish Washers can have even HIGHER Leakage Currents/Voltages due to their higher power requirements and need for SERIOUS EMI/RFI Filters.
There are SEPARATE "NEUTRAL" [WHITE] and "SAFETY GROUND" [GREEN] wires that run back to the Circuit Breaker Box and from there to the nearest Step-Down Transformer (where "Neutral" and "Safety Ground" are finally interconnected)....which means that that the resistance in the wires and DIFFERENT current levels flowing in each wire will almost always result is a small AC voltage difference between the "Neutral" and "Safety Ground" wires at any given electrical socket when under load.
It is important that the ELECTRICAL SIGNAL GROUND levels on all components are somehow interconnected, despite those crazy new connectors that are "slip-fit" without any secure latches or screw downs (e.g. HDMI, USB, Firewire, RCA). For those components equipped with a 3-prong power plug, Electrical Grounds are almost always interconnected via the AC Socket(s). For those with a 2-prong power plug...it's a bit ambiguous....provided mostly via device interconnection cables...many of which are NOT securely latched down. In my system, I have a separate Ground Wire that interconnects the AVR, Sub-Woofer Amp and whatever else that might need a more secure connection to minimize HUM.
First of all a few simple questions...sounds like something is missing in the Safety Grounding system:
1) What country are you in, so we have a frame of reference wrt Safety Grounding systems?
2) If the electrical plug on the Onkyo is a 3-Plug type with Ground on the third pin, is your house wired with 3-prong sockets? Which of your components have 3-prong plugs...hopefully plugged into 3-prong sockets?
3) If the electrical plug on some components is 2-prong, does the socket you plug it into have one spade wider than the other so that you can only plug it in ONE WAY? [Some OLD homes used very out-of-date sockets that allow you to plug it in EITHER WAY.]
4) Are all of the components plugged into the SAME circuit...i.e. all controlled by the SAME Circuit Breaker.
5) Are you using OTA Antenna and/or CATV Coax? Disconnect each and measure AC and DC Voltages between Coax Shield and a convenient "Safety Ground" point.
6) Did your X-Box come with a 2-prong or 3-prong Power Cord (both were made)? If 2-prong, and your sockets allow you to plug it in either way, try flipping it (and hence the Capacitors) around and remeasure leakage voltage.
7) With X-Box disconnected from Power and ALL other components and NOT touching any other component, set up a VOM to measure the AC Voltage between a convenient metal piece on a connector and "Safety Ground" and THEN (carefully) plug it in....sounds like you will verify that it's the source of your "Leakage Current" problem....and HOPEFULLY won't measure more than a few volts (but prepare for the worse, there could be an internal short circuit...SO DON'T TOUCH). Let us know what the voltage level is before taking any other actions. If it is a low enough voltage, the cure is to add a Safety Ground to a convenient METAL connection point on the X-Box.
Home Supply Stores have a 3-Prong AC Plug Tester that you might want to check to make sure that "Neutral" [WHITE wire] and Safety Ground [GREEN] on your Electrical SOCKETS are both functional (first remove all plugs from socket):
Or use a VOM to FIRST to identify which pin carries the AC Voltage (measure between two power pins and then between Safety Ground and power pins to verify which one is "HOT" [BLACK wire]. IF and ONLY IF there is ZERO voltage level difference between "Neutral" and "Safety Ground", it wouldn't hurt to also measure the resistance...however, DO NOT do this measurement if you see any AC or DC voltage level difference, it could damage the VOM.
North America uses either NEMA 5-15 (3-prong) or older 1-15 (2-prong) Sockets [NOT shown are REALLY OLD Sockets with 2-prongs that are the exact same size]...note that "HOT" pin is SUPPOSED to be on the RIGHT and SHOULD have a NARROWER opening that the LEFT. If you have the older 1-15 Sockets, esp. those with equal size openings....THAT could be the source of your problems....lack of the 3rd Pin Safety Ground:
Presuming you have a 3-prong plug on the AVR (or at least ONE component), this can be fixed by replacing the outlet(s) with a new 3-prong socket, making sure to connect the 3rd pin to the GREEN wire that is SUPPOSED to be part of your house wiring....unless it is REALLY OLD....so let us know what you're dealing with before taking any actions.
If you have electrical sockets that allow you to reverse the AC plug, reverse each component and remeasure it's leakage voltage....be very careful you don't inadvertently touch a "HOT" wire....are carefully read the WARNINGS section for each component to make sure it does not RELY on proper polarity for Safe Operations (let us know if they do). If in doubt...hire a qualified electrician....esp. if dealing with 2-prong Socket house wiring....