I recently swapped to using a multi-point powerboard with USB power also and noticed a faint hum from one of my synths. Knowing the sound was an earthing issue I started unplugging devices to find the source and it originated from the BSP, becoming louder when the purple lights of drum sequencer were illuminated. I've not tried the ground-loop eliminator yet, but I went back to using a USB power adaptor and removed the hum like that.
I remember years ago someone mentioning on another forum that different coloured LEDs on one of their instruments would induce hums, with certain colours producing worse noise than others. It was explained to me recently that LEDs produce oscillations that will interfere with audio equipment (why the ColorGameBoy had shocking sound compared to the old grey brick GameBoy originals due to the ColorGB's LED backlight). Now we all love coloured flashing lights on our electronics, but I'd happily go without if I could get cleaner sound (and why I used Nanoloop on a GB original).