Hiss is random noise generated by active electronic components. It is not induced nor due to grounding issues like a 60Hz hum. Nor is it impacted by cables, AC power or external electrical components. Are you familiar with ’gain staging’? Basically, your amps are running at max gain and your preamp isn’t required to run at a level very far above its noise floor. Solution: Reduce the input sensitivity on your monoblocks and drive your preamp harder.
As well if you are using a low-gain MC phono cart the signal to noise ratio of the MC preamp may be 10-20 dB noisier than a MM input. You can check this by switching to an unused line input, and if the hiss drops, that’s the contributor. The solution is the same - gain stage your preamp and monoblocks, and now your MC preamp as well, to raise the signal (variable) to noise (fixed) ratio.
Gain staging is a basic survival skill in live sound and recording studios, (it was one of the first things I was taught, along with grounding theory) but is rarely mentioned in HiFi. Here’s are two good good tutorials: https://www.izotope.com/en/learn/gain-staging-what-it-is-and-how-to-do-it.html
From Sweetwater Music: https://www.sweetwater.com/insync/gain-staging/