It's still noisy but not as much.
When the gain settings and connections are right, it will not be noisy.
…What if I a). use a TS cable from the synth into the H8000FW's analog ins, b) have the output levels back to 0dB and c) lower the inputs on the RME FF800 so that it doesn't clip?
Sounds promising – connect the TS cable from the synth to the 1/4" input of the H8000, not the XLR. Or, if you want to us the XLR, make sure the cable is wired pin 2 to tip, pins 1 AND 3 to sleeve.