Intermittent, random, loud popping noises.

The connections should tell you that. If it has sockets for two sets of speakers and two inputs for the sound then it will be a stereo model.
What you can do to test the system is to disconnect the speaker connections to the receiver and use a 1.5 volt battery connected across each speaker cable end. For only a brief time. At the other end you should hear the speaker pop. I suspect you will need two people to do this. Make certain the volume control unit is on full.
If you don't hear the sound of the pop the cable is either faulty or disconnected somewhere.

Volume controls are known to go intermittent and cause this.Its a surface mount type and would require exact part.Try removing back of pot to get inside to chemical spray it out with pot cleaner.

Check the sub with a multimeter. It should read the ohms it is rated at when you hook it to the positive & negative inputs. It its off your coil if bad.

Check the impedance (resistance) on all the speakers and the speaker wires themselves.
Power up the unit with no speakers or inputs attached at minimum volume and see if it still goes into protection mode.
Most speakers should measure between 4 and 8 ohms for most systems when disconnected from the receiver.
If the unit says PROT even with no inputs at minimum volume with the speakers detached, then there's a problem with the amplifier itself, probably in the output section.

I had this happen to my unit shortly after I purchased it. When I turned the receiver on there was a loud pop through the speakers, the red light on the front started blinking and the unit failed to produce sound.

A tack worked its way through one of the speaker wires, causing a short. I replaced the wire and unplugged the receiver for an hour. It's worked well for the past two years since this issue.

If you haven't done so already, check all of your speaker wires and plugs on the receiver and the speakers. You may have a damaged wire or metal-on-metal contact somewhere. Try these steps:

1. Unplug the unit and disconnect all wires and connections. Let the unit sit for an hour.

2. Check all cords for damage or crossed connections.

3. Reconnect the power cord and set the volume to minimum. Turn the receiver on. If it stays on, shut it off, unplug it and reconnect everything, taking care to ensure that speaker wires and/or plugs don't make contact.

4. Plug the receiver in, set the volume to minimum (it should already have this setting after step 3) and turn it on.

It sounds like there is a short somewhere in either the wiring or the receiver. This caused the popping noise and activated the receiver's protection circuit. If it's a damaged wire or plugs or wires making contact, you can reset the unit with the directions above, replace or separate the wires and all should work. If the issue is within the unit, you'll have to contact Onkyo directly.

What models of speakers all around? Certain combinations of speaker efficiencies won't be within this unit's capabilities to compensate the volume.

Starting on Page 17 they discuss how you can adjust the Digital Acoustic Processor (DAP) mode to modify the levels of the rear speakers -10dB to + 10dB.

Starting on Page 20 they have similar settings for 5.1 discrete analog inputs.

just repaired my se 501 poor solder conections on blue rleays by speaker jacks,also blown center speaker seems there are 2 in it one blew and so did cpacitor running now just hooked up one in center speaker sonds great also stay at 8 ohms setting

you have a problem with your amp. most likley the finals are passing dcv intermitently to the load.

you may be using an equalizer? try to pop-up the volume with all tone controls in flat mode. see if it still cuts off. if this is the first time youencounter this then something must be wroing with either your speakers or your amp. unless your ventilation is not good enoughg, you can also try placing a blower on top of your unit to cool it down

i think I know what your problem is ... if you hold down the menu/select button ( the button between the volume buttons) there will be a volume option it'll say P --- volume. I think that sets a default volume that the cd player will always reset to once the cd player is turned off then on again.