Just something I spotted while browsing the Audio Note site ( as you do ) looking at the DACs.
I hadn't spotted before that the DAC 4 'Sig' and DAC 5 's use 6463 double-triodes ( in PP ) in the output stage rather than 5687's .
http://www.audionote.co.uk/products/dig ... l_01.shtml
Of course they are NOS Telefunkens - I wouldn't expect anything less

On paper ( Philips datasheets ) they don't look especially linear, although it may be that they have less odd-order distortion than 5687's ( which don't have much anyway) , or something subtly different in the spectrum .
I don't know if you ever tried them in anything, Nick ? - I notice they are incoveniently a different pinout, so it wouldn't be easy to swap them into an existing DAC unit . They do appear to be similar to 5687 in characterisitics otherwise.