I suppose the amp has a low-frequency hum coming from the chassis, not a high-frequency hiss? This is quite normal and depends on the mains quality. It can (and often does) vary during the day. As long as it is not too loud it’s not a cause for concern, you can read everything about it here:
It’s most likely not related to the hiss.
Regarding the speaker cable, it’s not so much about high quality but about whether it has the correct electrical specs at the short lengths you are using:
Do you know the specs of your cables, i.e. inductance and capacitance? I found nothing on the extrasound website. If you don’t know, what is the construction? Do you have a photo or the model name?
Like I said, the hiss that you have, even when muted, and so audible at 3 meters, does not appear to be normal as far as I can tell