Can I plug a transformer or stepped down power supply rather than mains voltage to run my K8000?
It has never been tried but most likely yes, however, you will loose the galvanic isolation between the K8000 and your PC.
You will need a 12VDC supply.
Where do I connect a 12DC to? I see only a mains input
There is no 12VDC input.
You will have to remove both transformers and feed 12VDC directly to the ‘secundaries’.
Sorry, we have the impression that you lack knowledge to perform this modification. We strongly suggest you seek local assistance, as we cannot assist with modifications.