In this Gear Page thread I suggest folks running the POD HD500 as a pedalboard before an amp can reduce hiss and get a better signal to noise ratio by making sure the mixer pan controls are both set to centre.

I've been fiddling around with HD Edit and headphones this evening and I came up with some other things I thought worth sharing away from that thread, because they apply whether running amp models or just FX.

It seems a lot of the hiss you may be hearing is actually coming from inactive inputs adding noise into the signal chain. Most default empty patches seem to be set as follows:

Input 1: Guit+Aux+Varx Ch1

Input 2: Same

And this seems to introduce hiss and unwanted noise. Now try the following (assuming you've one guitar plugged into the regular 1/4" input and are not using a Variax):

Input 1: Guitar

Input 2: Mic

then:

Input 1: Guitar

Input 2: Aux

and finally:

Input 1: Guitar

Input 2: Variax

If you're using a patch with some gain in it you'll certainly notice hiss, decreasing as you try each example above. With the final one Guitar/Variax being the quietest.

Now from that setting try:

Input 1: Guitar

Input 2: Guitar

Notice how much louder your signal suddenly gets? (You'll notice these examples more with headphones on). Perhaps this is one of the reasons some folks have been having issues with clean sounds not being as clean as they hoped. Setting Guitar/Guitar seems to put your signal through stronger. For low noise and low hiss it seems that you're best setting the HD inputs to 1: Guitar / 2: Variax (or an equivalent if you're using a Variax - the general principle being to use a second input that doesn't have anything actually running into it).

Now going back to my suggestion in the previous thread, I believe I was wrong when I said centring the mixer pan controls isn't relevant when using the amp models. I believe that the signals don't seem so much quieter when panned compared to FX only, but I believe it's the setting of two guitar signals as inputs that compensates for that so the earlier principle of centring controls still stands.

So what's the practical upshot of all this waffle? I've just gone through all my patches on HD Edit and centred the mixer pan controls (double clicking them does this quickly). Obviously you won't want to necessarily do this on patches where you've placed FX/Amps in the left and right channels, but try it on the ones you haven't  I've also changed every patch so that the inputs are set to:

Input 1: Guitar

Input 2: Variax

I chose Variax as this offered less hiss than an unused Aux or Mic setting and I don't have a Variax. Line 6 really needs to add the option to switch inputs off completely rather than allowing them to add noise into the signal chain.

Anyway the result for me of changing all the patches to (1) Guitar/Variax input and (2) centred mixer pan controls is that all the patches are much less noisy and feature much less hiss. This seems to be a very good solution to some noise issues for me - and seems to result in a more pure signal chain from which to begin building patches. It seems foolish that Line6 has the empty patches defaulting to settings that introduce noise into the system.

I hope my experimentation helps some of you.

PS: Those worried about losing stereo separation by centring the mixer pan controls needn't worry. They are only for setting the stereo position of FX within those left/right channels. Any stereo effect after the mixer works in stereo as normal whatever the mixers settings are.