The gain knob will introduce more distortion the louder you turn it up. I suggest keeping it as low as possible to prevent damage to the speakers. I usually don't tweak mine over the 2-3 mark (20-30%)....if I need louder, I buy an amp with more wattage.
As for your old amp - take the heatsink cover off. Most amps come with an external fuse (car type fuse plugged into the chassis or an inline fuse on the power line) and often times will have an internal fuse on the main board - typically a glass fuse. Look to see if it's blown and replace.
For more info to understand gain, here:
http://whatis.techtarget.com/definit...499581,00.html
I've found blown fuses in lots of home stereos that people have given me .... 35¢ and I have a working stereo.
Just as a note - turning the gain up too loud will lead to "clipping" which is a term used to describe overload on an amplifier - it clips on and off, causing interruptions in output. I would never pump the gain over halfway.
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