I should point out this feature is not a gain adjustment. It is in fact a digital bit truncation that was implemented because the Cirrus Logic CS4398 DAC chip used in BCD-1 and BDA-1 has a higher output voltage than usual. In case your preamplifier has a problem with it, you can reduce the output level by 3 dB. This is an inherent feature of the CS4398 as a matter of fact. I think the datasheet stated it allows for a -60 dB level reduction but Bryston might have limited it to -3 dB to preserve audio integrity.
I don't think there would be a noticeable drop in performance if you reduced the level by 3 dB but I would still rather use the volume control on the preamplifier.
This is also the reason why BDA-1 has a higher SNR than BDA-2. It is also the reason why CS4398 is favored among tube enthusiasts as it is a voltage output DAC rather than a current one.
Cheers!
Antun