OK I have had a chance to listen to some BNC cables. I have a 0.5M BNC to BNC, a $500 audiophile BNC, and a 2M BNC to BNC.
To remove variations due to cable upsetting other things, I used MSI lap-top > USB > Blu 2 > BNC cable > Dave > Audioquest Nighthawks.
The first test was the 0.5M against the audiophile cable - and the $500 was smoother, with better depth.
Next I tried the 2M BNC against the 0.5M BNC - and the 2M was better than the 0.5M cable. Moreover, it also was a bit better than the $500 audiophile cable.
The longer length is perhaps surprising; until one realizes that longer lengths means more RF filtering by the cable itself.
So I have on order some more cables, of 1M, 2M and 5M lengths, together with ferrite clamps on cores as this should further improve RF isolation. I will report back.
Conclusions? Yes there is a small difference; so far don't spend huge sums on audiophile cables, just quality RF cables.
Rob