1. Pre-amp RCA --> power amp RCA in, you can use normal cable without mod.
2. Pre-amp Balance out --> power amp RCA
Take + and GND to power amp RCA is OK
3. Pre-amp RCA to power amp Balance
Add a circuit to invert the +ve to -ve.
i.e. RCA --> circuit --> HOT GND COLD --> Power amp XLR.
You can use TI DRV134 un-balance to balance op-amp