I can hear the hissing and humming noise when I keep my ears close to speakers, so is it normal that any preamp will produce this kind of noise (even without any input cable)? Or is it an issue with my item? Sorry I am a perfectionist.
Completely normal. Has to do with the minimum gain on the preamp, your amp sensitivity, and your speakers. I would not change anything but you can put a pad on your preamp cables and that will knock it down. I also keep a pair of attenuators around just for this problem.