It turns off when the volume is turned up over one third of the volume knob, but is the speakers don't interfere in any way

Sounds like the protection circuit is the problem here which is a good thing because its stopping you from permantly damaging the unit first thing to check would be the impedance of your speakers and then check what your impedance selector is set to on the back assuming you have one be warned that you should never move this switch while the unit is on.My advice would be to unplug it first because that switch plays a direct role in the systems power supply..Hope this works for you

It's the protection circuit that's shutting the receiver off.
The receiver output are being shorted out.
Here is what you need to do.

1.UN hook everything and all speaker wires from receiver.
2.Make sure nothing is hook to the receiver.
3.Turn the volume all way down.
4.Plug in the power cord and turn the receiver on.
5.Put the receiver on FM making sure its on a radio station.
6.Slowly begin to turn the volume up while listing very close near the receiver.
If you here a click or see the display flash OVER LOAD while turning up the volume then your receiver is in need of service.

If you don't here a click, or the display doesn't change then the receiver is good to go,and you need to change speakers.

Do this and you will be happy!!!