I own a Logitech Z-5500 surround system. To save energy, I always disconnect it from electricity using a master switch. Unfortunately this resets all the settings (choice of input channel, volume, playback mode etc).
Over the years, it happened once or twice that all of a sudden the system started up with different settings. I could not figure out what the cause of this was. Until I stumbled over this forum thread.
There are several easter eggs in the control station of the speakers. One of them is a "second skin", which enables an alternative display style. I never actually used it (I prefer the default one), but I remember that I played around with the different settings that the easter eggs provide. Something about that must have had a lasting effect on the settings.
Turns out it was the second skin. By enabling and disabling this feature, the settings are stored even if you disconnect the system from electricity.
First of all, configure your system how you like it. Then put it into standby by pressing the power (1st revision) or headphone (2nd revision) button. To enable the second skin, press the input + effect + settings + mute buttons simultaneously for about 5 seconds. The backlight should now turn on and display Second Skin On.
Then disable the second skin by putting the device into standby again and pushing the same buttons simultaneously for another 5 seconds. The display should now show Second Skin Off.
Your settings should now be stored permanently. To change them, repeat the process above.