Very faint hiss from tweeter

The Schitt freya + if using tubes the hiss is from the tubes ,on position 2 SS mode on preamp it shouldnot do this.

I concurr with lowering the gain on the Parasound. My A21+ gain is set at two o'clock (two thirds). Not for hissing, but I get the best sound there.

Due to having ancient, ungrounded circuitry, I found it necessary to go against  protocol and plug my integrated amp into a line conditioner. When I turn the volume knob on my entry level amp (Cambridge Audio CXA-81) through my equally entry level speakers (Revel Concerto2 M-16's) well past 3 O'clock, I get no hiss. Even on the line input attached to an entry level phono preamp, I get no hiss and only faint hum well past 5 O'clock. Could you be experiencing RF or EMI issues? The last time I experienced hiss through any tweeters was when I using a well worn Rotel preamp.

Move the modem and router to another room or farther away. And buy a puppy

My system is stone-cold, dead quiet with no audible hiss from any of the speakers set at any level.

BUT, I’m using all SS equipment not tubes. Tubes are notorious for being noisy. Especially cheap tubes.

Could also be "dirty" power or noise somewhere on the line.

If you’re bothered by it, indeed, try some cable management. You could also swap out the tubes for another brand (I quite like Electro Harmonix’s Gold ones, but have also had good results with Genalex-Golden Lion and Mullard’s upper end offerings).

You’re probably in a "try it and see mode".

From Schitt’s website:

Whisper-Silent Tube Stage—With Tube Shutdown

Now, you can enjoy a tube preamp without the hiss and hum of classic designs. Freya+ offers super-quiet tube modes with DC heaters and semi-circlotron, noise-cancelling output stages. Better yet, the tubes turn off when you’re not using them! That’s right, both the tube heater and high voltage rails go away when not in use, so you can preserve lifetime of costly tubes.

Good luck.