Broken power switch on Sony STR DE725

Simple, if you are not confident in your ablility to fix the problem by dissasembling the boards then by all means, take it to a repair facility.

Yes, it will support a powered subwoofer.

If your existing speakers are from an all-in-one Home Theater in a Box and they have molded connectors you'll have to chop them off to make the ends bare wire and determine which is plus and which is minus on each. Loads of 4 ohms may present a problem at high volume levels.

Take the rated wattage of the sub and other speakers in an HTIB with a HUGE grain of salt. They plays numbers games with th low impedance and the allowable distortion levels to get the results they publish.

For all anyone knows, the speakers and their original electronics may have been designed to operate together to yield adequate performance (like Bose does). Breaking that pairing may give unpredictable results and risk of damage to the speakers or the new receiver.

check to see if its switched to stereo mode

It sounds like the switch might be OK mechanically but not electrically. Have a look to see if you can check the connections on the switch. Perhaps there is a broken or loose or bad solder joint.

sony are pretty rubbish at offering patent parts, my suggestion as poor as it is would be to approach sony for this part they will recommend a retail outlet that can supply it

did the left channel work when u switched speakers, if so then the left speaker or wiring is a fault.if not then the output circuits on the amp are blown or the fuse is blown some amps have internal fuses in the output to protect the output transistors. unplug the power cord and take the cover off, look to see if u can find any fuses and check these for continuity u can hold these up to the light and u should see a fine filiment in one piece and not broken. these fuses sometimes blow just from age and use at high volume like a regular light bulb.If the fuse is blown replace with the same value else the output transistors will need to be replaced most likley along with the drivers.

Go to this site (sony)
http://esupport.sony.com/US/perl/model-documents.pl?mdl=STRDE725

The sony device may not be able to support the Toshiba. Make sure that is completely compatible and that you have the correct cables and setting to support everything.

My sony used to do that to umm cant remember the model but yeh i could play it at low volume but as soon as i turned it up it would start clicking and telling me to turn of the power...

I resolved it by checking every cord connected to it, it turned out that because I connected two speaker cables together i didnt seal it properly and they were touching this was making it go into protetion mode.

Here is Sony's support site for that model.