So I had an ARC preamp like that (Reference 6) and in fact used it with its RCA inputs (from a phono stage with only RCA outs) and XLR outputs to my amps - and though that sounded best (compared to RCA outputs). Try it both ways, but my "guess" would be that XLR between preamp to amp is likely to sound best. You’ll usually also get +6dB extra gain this way, so make sure to adjust volume accordingly.
I also have VAC Master and Rogue Hera preamps with this arrangement too - RCA & XLR ins & outs. The Rogue and ARC are doing something to the RCA inputs so that it's a balanced signal before it hits the doubled (balanced) circuitry. I think the Master might actually be a SE circuit with full transformer-based I/O to facilitate the flexible I/O.