Thanks for the quick reply. My interest is in using sound cards for intrumentation, hence the wish to avoid low level artefacts introduced by software layers, even if not audible.
For information, all 3 sound cards advertised 24 bit conversion. For two of them (the third was proprietary) I downloaded the codec (A/D & D/A) data sheet some time ago, and confirmed that it was designed for 24 bit operation, although system noise will restrict the number of usable bits to perhaps 18. For the only card with an ASIO driver I have run a managed ASIO interface (an extension of Rob Philpott's library) and confirmed that noise level drops by a few dB when switching from 16 to 24 bits, as do low level artefacts.
Apologies if I am posting this in the wrong forum, as NAudio is blameless, but others may like to know that shelling out for a 24 bit card won't necessarily get them 24 bit WASAPI.