I don't have a National, but I do have a cheapo copy (Ozark) that sounds very good (once I'd sorted it's bridge out). I play mainly blues slide on it, generally in open D, occasionally DADDAD, and even more occasionally open D minor.

They are, to me, the ultimate guitar for slide.

Bob Brozman is something else isn't he? Got the chance to see him last year - inspiring.

Pete.