A number of different methods were employed when AVA first started looking into support for XLR. This took place between late 2016 and early 2021 (if the dates on my files are accurate). In the end, we decided on the approach that fit in best with the amplifiers we build: XLR for bridged monoblocks and RCA for single ended stereo. This connection method results in no "extra circuitry" in the signal path, and no "extra power supplies" required. In the event that a user wanted to use monoblocks and only had single ended RCA source available, the R2X was created and made available.
Any stereo with XLR inputs, or more likely switchable XLR / RCA, uses op amps, relays, and a plus and minus 15v power supply to accomplish this.
Dan