Thanks for all the advice, I think I'll try to make my own cable adapters with quality components on the connectors. I was thinking of using Neutriks XLR NC3FXX-BAG connectors and Neutriks NF2C-B/2 phono RCA with Supra cable, but not connecting ground on pin 3.
I don't think there's a need for a converter as the cable length is only one meter.

XLR Pin Connects to RCA
Pin 2 (Hot) RCA Centre Pin (Signal) The audio signal itself.
Pin 1 (Ground) RCA Outer Ring/Shield (Ground) The ground reference.
Pin 3 (Cold) UNCONNECTED (Floating)

Have tried to find characteristic data on 861 and 258, I have found this data, is it correct?

The output impedance is 47 Ω on the Meridian 861 XLR output (balanced OA18)
The input impedance is 16 kΩ on the Meridian 258 power amplifier (Unbalanced)

861 Preamp XLR Output (Balanced OA18) 4.5 V RMS
The voltage will drop by approx. 6 dB, i.e. from approx. 4.5 V to approx. 2.25 V RMS in the transition to unbalanced.

258 Power Amplifier RCA Input (Unbalanced) 1.0 V RMS Nominal level to drive the amplifier to full power.

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