A while ago I was having an awful time with noise using my HD500 recording the s/pdif output.
I got lots of advices and links and it did help greatly.
Then I picked up a used Peavy Valve King Royal 8. So noisy and it sounded lollipope thru the craped out 8'.. I installed a speaker ext jack and use my old not working Spider II as a speaker cab. Big improvement but the Amp was still noisy.
A few weeks ago I picked up a used Isolating transformer for silly money from an Auction and l got to tell you how quiet things are now.
This thing really cleans the line and filters it as well. Basically the transformer unlike a stepdown or stepup maintains the same voltage and like all transformers isolates the input into the primary coil by inductance pickup in the secondary coil. This one has filtering thru the common earth.
Not to be confused with a Power Conditioner which are over priced power strips imo that offer filtering and surge protection but alas noise on the AC still gets in unless it contains an isolating transformer. Expensive too.
The Amp is now quiet the guitars are now quiet, even when hands are off the strings and the POD HD is bearable using high gain amp and Distortion models.
Do yourself a favour and get yourself one for the Studio and while heavy, worth it on the road too!