1st step, you'll need to identify which revision of the board you have.
If you are using speakers while you are testing, it would be a good idea to connect a series resistor, maybe 100R, to save a speaker meltdown (not to mention your ears). In any case I wouldn't use any speakers you care about
Alternatively, buy some power resistors (I use cheap 15R / 5W) and connect them in parallel with the speaker + 100R resistor so your amplifier has something to push against. Wound power resistors are very robust so no worry about only being 5W, they will probably be glowing red before they die!
From the sound of it you have at least 2 different faults, 1 channel with 'hiss' but otherwise working and 1 channel with almost no output. On the other hand, you have no exploding fuses or smoke being emitted which is a good place to start.
I would start by looking for any dc offset at the outputs and ripple on the power lines.
dc