Turntable with built-in preamp buzz/hum

Hi, I bought a turntable with a built-in preamp (Pro-ject Primary E Phono) and a set of active speakers (Edifier R1000T4) so I could just plug-and-play. Everything seemed okay and sound was good to my ears, which are new to vinyl. I was using the RCA cable that came with the speakers but it was short so I upgraded to a longer QED Profile cable, which is supposed to be a good make of cable. Everything was fine for a couple of listening sessions but then a loud, sudden buzz that fades back to a hum started happening every time I stop the turntable from spinning. Even if I don't play a record and just start the turntable spinning and then stop it the buzz and then hum happens. I've reconnected the RCA to the turntable's Line Out and the speaker's AUX In and I've reconnected the speaker cables to the spring clips. What's strange is that it's less harsh if I use the "cheaper" cable that came with the speakers. I would've just gone back to using that cable if it made the issue go away but it's still there, just not as harsh. The turntable has an earthing post for when Phono Out is selected but it doesn't say that's it's needed when Line Out is selected. In any case, the speakers don't have an earthing post. Sound still seems good but it could just be because I'm listening out for problems that I'm focusing in on any funny noises I hear when playing records. Any guidance is much appreciated. Both the turntable and the speakers are plugged into the same extension lead. I haven't added any new electrical equipment to the room, either. I wonder if the problem was always there and the "higher quality" cable has shone a light on it?