Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
You hear hissing in the speakers when the amp is turned up, because you are amplifying the noise that all amplifiers create. The trick to making that hiss disappear is to make sure your incoming sound signal is actually stronger than the "noise level" of the audio amp.
I don't know what your source of audio is, but I do know that I hear hiss mostly when the amp's volume control is turned up high, but the audio source has its own volume control and it's turned down.
An example that I see a lot is cable boxes with their own volume control for the box.
I find those volume controls set really low, and then the amp's volume control set really high and you hear that hiss.
Most other equipment, like DVD players and Games and VCRs don't have a separate volume control on them, and their audio source is always at maximum. But cable boxes are known to create hiss in the audio because their volume control is turned way down.
Of course if you have no audio coming from the unit, you will hear that hiss too.
So the question I am wondering about....is the hiss coming from a cable box and if so, have you looked at the front panel of it, found a set of buttons labeled Volume + and - and press the volume + button and held it until the sound was set to maximum?
I always set that volume to maximum and then I go to the amp and set its volume level so it's a little louder than the customer usually uses. That way, if the customer is actually turning down the volume on the cable box instead of the amp....they have plenty of range to adjust it for normal listening levels and you lose the hiss.
I kind of suspect that's the reason why, but in all honesty it's wise to know if that hiss only comes from that one unit, and find out it's brand name and model number. Those are usually found on the front or even the top edge of the unit. Rarely do you ever have to look at the back to get the model number on Cable boxes, satellite boxes (sometimes hidden behind an access door on the front of those) or DVD players.
Also, the wire from that box that connects to your Panasonic amp.....you happen to know if it's the optical cables, or one set of the older RCA audio cables?
This is a set of RCA --