I have purchased a 258 power amplifier that I want to use for rear and side channels, but I have two OA18 cards in my 861v8. Can I use an XLR to RCA Phono adapter cable or do I have to buy an OA08 card to replace one of my OA18s?

TV room: 861v8, 800v4, UHD722,Front 2xDSP7200SE, DSP7200HCSE, Side DSP5200, Rear DSP33, DSW, (DD 15 Sub),
Roon Rock NUC, QNAP NAS, Chromecast Ultra, Apple TV, Oppo BDP-105D, Pro-ject Pre box S2 digital, Philips Pronto remotes, Supra LoRad Power.
Office: 800v3, MS200, DSP3100
Basement: MS600, 502, 557, Dynaudio 52se, Dynaudio Sub 250