If every test is as you say, I'm baffled. The ONLY answer is that a) its the amps and b) they both changed at the same time. And suddenly.
This is very unlikely.
Speakers don't generate hiss.
Passive components don't generate (meaningful) hiss.
You removed the preamp.
You swapped mono amps with no impact (are you sure?)
It is coming up to Halloween and a Full Moon....
This is very unlikely.
Speakers don't generate hiss.
Passive components don't generate (meaningful) hiss.
You removed the preamp.
You swapped mono amps with no impact (are you sure?)
It is coming up to Halloween and a Full Moon....