DJM450 Hissing Sound

I have recently built my first set up - I have 2 x Technics SL1200s, paired up with the DJM-450, a Marantz PM6005 amp and B&W bookshelf speakers and is primarily for domestic use.

When I put it together, all was well and has been fine for a couple of months, but yesterday when I turned it all on, there is a loud hiss coming through the speakers. It starts as soon as I turn the mixer on and have the master volume at any level above zero. I can play records and the hissing seems to reduce, but can still be heard in the background underneath everything...

I've tried everything I can think of... I've removed the decks from the equation by plugging and grounding each of them individually to the amp and they work without hissing. I've also unplugged everything from the mixer and have the mixer connected to the amp with analogue interconnects and it's still hissing - so there are no inputs plugged into the mixer at all and it's causing the speakers to hiss...

Any thoughts...?