As @BobTFM35 pointed out, you definitely want to replace that power plug with a grounded plug as it originally was configured. A previous owner was likely trying to get rid of some hum in their setup and starting lifting grounds to see if that helped. This is not really a good idea but sometimes in the field (usually as a last resort), grounds start getting lifted to attempt to eliminate a ground loop.
As to the connection between the C1 and the PM1201, you'll be making an unbalanced connection so you can use either the 1/4" inputs or the XLRs on the amp. You should be able to find pre-made cables for both at Amazon. I like to use XLR connectors as they make the best low-level connections. So the cable would typically be RCA tip to XLR pin 2, then connect both the shield and ground lead to the RCA ground, then the ground to XLR pin 1 and the shield to pin 3. Make very sure that you configure both channels the same with respect to pins 2 and 3 so you don't get one channel out of phase relative to the other.
If you're making these cables up yourself, you'll need a 2-conductor cable with a shield - 100% shielding is always best and something like Belden 8451 works nicely although it has a hard PVC jacket. For something more flexible you can try a mic cable like Mogami W2549 or Belden 8412.
Good luck with the new setup. Should sound very nice - what speakers will you be using?