6505 +112 has a bad hiss with nothing plugd in

Swap channels to verify the problem stays with the channel and is not a function of the mic or cable.
If the problem stays with the channel, it is likely one of the RF bypass caps at the input is failing. It could ALSO be a bad solder on the XLR connector into the board... in either case it will be a PAINFUL repair to get at the problem unless you can re-solder the connector from the bottom if it is the problem.

The source of the noise could be many things. At real high volume levels with no notes being played, some hiss is common due to the statistical properties in electronics. At a normal listening level the hiss should be inaudible. If there is a failure there is little you would be able to do yourself to repair it. One MIGHT use circuit cooler to isolate a bad component, but it is unlikely you would have the tools to replace it as most everything is surface mounted. If you have a treble control, turn that down to reduce the level as the hiss is always up in the higher registers.

It is likely that there is a failure of one of the amps in the unit. It could be a cracked circuit board, bad solder, bad volume control, or a bad compnent in the power amp section for one side. HOWEVER something does NOT compute here as you can't connect more than one speaker to a 15 Watt Crate amp... Maybe you have two of these amps.

If you are having the treble/high turned up there will be hiss. This is due to the statistical noise in electronics, including semiconductors. It is a matter of degree. The hiss should be 30 Db below your program level or so.

It is very common for one to change settings and suddenly start to notice hiss or hum at certain settings.

IF YOU CAN, go to a music store and COMPARE the amp to yours at the same setting with NOTHING plugged in. (cables and guitars can generate hiss or other noise themselves).

There is very little in your amp that would have changed to suddenly start generating hiss, so FIRST compare. This will minimize the chance of spending money at a shop to be told it is normal.

This could be many things. It is most likely noise from a defective chip in the unit. If sounds are normal except for the hiss, then test if the volume control reduces it. IF not, then problem is in the power amp section. If it does vary the loudness, then problem is likely between the D-A convertor and the volume control.

I would first try to isolate this by cooling selected parts with circuit cooler mist to see if you can find one that reduces the hiss when you cool it... Spray, then wait ten seconds before moving to the next on.

USUALLY semiconductors cause this, but rarely a bad resistor or cap can as well.

This is likely in the power amp or the drivers for it.

The key to troubleshooting is a schematic. Try to procure one.

You MIGHT try using circuit cooler in the area of the power amps to isolate a bad component or cracked circuit board.

The hissing is caused by a bad connection, most likely at the cable jack. Try replacing it and while you're doing that check the rest of the components - especially the potentiometers, used ones generate a lot of static.

A certain amount of "hiss" is present in electronic amps due to the statistical noise in electronics.

The hiss should be WELL below the level you get when playing. If you put you ear to the speaker, expect to hear some hiss with nothing plugged in. Ten feet away it should be very faint.

A relatively quiet "hiss" is normal and is due to the statistical properties in electronics. If the level is noticeable during normal program play, then it needs to be dealt with.

First thing is to verify the noise is still there with NOTHING plugged into the input jacks. Many times the noise comes from bad cables and guitars that have noisy on-board electronics, so we must check this out by unplugging all inputs.

MAKE SURE any other devices connected to this that require connections to the power input ALL are plugged into the same receptacle.

The slider or the amps for channel 1 may have a problem.

Sliders do wear out.

Try shorting channel 1 input and see if that affects the noise. A hissing noise is probably a bad preamp chip. That is shop work at a repair facility.