Your unit is likely shutting down because you have a short on one or more speaker outputs. This is a safe mode on short circuit to protect the unit.
The hiss is an indicator of the short (it has been shorted so long that one transistor set is probably bad. Look for a single strand of wire crossing from red to black speaker output on the receiver or at the speaker input on the speaker. If not there - you may have touched speaker wires together at one time - especially when volume was turned up.
Most likely you have blown the power transistor to that channel (left). You will have to open up the unit and look for the large transistors mounted to a heat sink (large aluminum frame). There should be two transistors for each channel (12 total for six channels) in an AB (push/pull) configuration.
You will have to measure the output (AC voltage) of each pair - right wire is usually the emitter. You can hold the neg (black) lead on the volt meter to the black speaker terminal when checking each. If you are lucky - there will be white lettering to indicate which speaker each pair of transistors is for.
Simply desolder the bad ones and replace with identical type (www.digikey.com
should have most transistors you will need - radio shack will not have the correct transistors). If you get no AC or only low AC or AC output voltage does not track with volume knob turn, then you have a bad set of transistors on that channel. Compare to other channels.
Be EXTREMELY careful when you open the unit. Static electricity can damage components. Touching something near the AC main (wall power) kinput, the filter network for AC main power, transformer, rectifier will damage you. If you are uncomfortable, you can take your time and learn how to do this (Books, videos, learn from a friend). Do not do this without knowing about electricity and where the hot spots are in your unit. You will be much better off spending $300 - $500 on a new unit rather than risking your life/health to fix this unit.
Good luck and drop a note on this site to let me know what you do.