Call me an old cynic and obviously I am not a technical expert but
doesnt simple logic mandate that when you sub a speaker and the
sound goes away, all it means is that (what I thought blindingly obvious
from the symptoms and there being no apparent logical connection
between the source and the hissing, which was why I posted he
Because "The problem is that you are describing something that doesn't
seem to make any sense, or have any obvious explanation" so I wondered
whether anyone had come across it before) the sound wasnt coming from
the source? Or did I have to add that when i put the speaker back
again, the sound came back?

Your speakers are passive devices. They cannot make any sort of sound on
their own.

If hissing is coming from one of them (but not the other, when driven from
the same amp), the speaker must be therefore be the source of the hiss. But
as there is nothing in the speaker that could, by itself, create sound, then
the speaker must be altering or distorting its input to produce the hiss.
However, you say that the presence or level of the hiss is not related to
the program material.

This makes no sense. The logical conclusion is that A: you are grossly
mis-describing the symptoms, or B: this is a troll.

I think it's B. I really don't have any more time for this tsuris/tsimmes.
Take your pick.