I don't have RCA connected to my HPA4.
I only have HoloAudio May DAC KTE which outputs 4v on XLR thus I've added +10db on that input.
Rory Rall from Benchmark wrote to me on a that same question:
"The HPA4 is at unity gain at ‘0’ on the dial.
I cannot find a spec sheet on the Holo but my guess is that its XLR output is around 4V. If you have the AHB2’s sensitivity switch in the middle (4V) setting, you’ll not reach full output of the amp until you are at ‘0’ on the volume dial. So if you’re using it at 80-90% of the dial, this is correct. You’re normal listening is about 10-20dB below full output.
Suggestion: Verify the output of the Holo. If 4V, add 10dB of ‘boost’ in the HPA4 setup for that input. Then, set the AHB2 switch all the way down to 9.8. You’ll then reach full output of the amps at about -2. It really doesn’t matter where you are on the volume dial, it will be clean."