Very likely a ground loop complex topic why don’t you Google and read up on ground loops and then do a bunch of experiments to identify it and eliminate it also how are you driving mono block amplifiers with a phono stage and no volume control in between them seems to me you’re leaving something out of this
Ground Wire
The tonearm cable is plugged into RCA input of the phono stage. The tonearm ground wire is connected to the ground post of the same phono stage. The first RCA L/R outputs of the phono stage are connected to a pair of mono blocks. There is no hum I could hear coming from the speakers connected to them. The second L/R outputs of phono stage are connected to the line input of the integrated amplifier that drives a second pair of speakers. That's where the hum is quite audible. That integrated amplifier also has the ground post. I wonder if I could get rid of the hum by connecting the phonostage's ground post with the ground post of the integrated amplifier. What kind of wire should be used to connect two ground posts of two different preamplifiers? The units are at least 8 feet apart.