Interestingly I have access to a couple of Sony universal disc transports, which handle CD, SACD, DVD, BluRay and Ultra BluRay. Each transport has two HDMI outputs, one purely for audio.
Each is powered by a 12-Volt wall wart, so it would be very easy to substitute a 12-Volt car battery for the mains supply and do some A / B comparisons.
What objections are likely to be raised by the community, a) if there is no clear audible difference or b) if a clear audible difference is reported?