Unless this is just a suggested specification, check out page 10 of this pdf file for the pre-emphasis, and page 12 for the de-emphasis:http://www.nrscstandards.org/SG/NRSC-1-A.pdf
I remember an engineering friend who told me after NRSC came in, he could no longer stand to listen to the AM radio in his ('55? '65?) pickup... everything was too bright.
(I'd gotten the impression some stations had been boosting their highs a LOT more than 10db, so this was a bit of a compromise at the time!)
I find most AM stations sound a little better in wideband on the Superradio III with the treble dialed back a little, but it's not horrible.
EDIT: re-reading your response, I think we may be talking about nearly the same thing. The cutoff is basically 10 Khz (9.6, close enough) and drops like a rock below that.
A little more research has shown, though, that the NRSC mask IS a requirement for all U.S.A. AM radio stations:
NRSC-1, published in July 1998... A modified 75 u/sec preemphasis/deemphasis curve was recommended, much like the long used system used in FM exciter and receiver designs. In addition, the standard recommended limiting audio bandwidth to 10 Khz (-30db @ 10.5 Khz., -40db @ 11.0 Khz., -50db @ 15. Khz.). This "audio masking" scheme reduced percentage of modulation at 15 Khz. down to as little as .32 percent... NRSC-1 was a voluntary standard.
NRSC-2, published in June 1998, further described the reduction of AM broadcast spectrum bandwidth as a product of limiting audio frequency bandwidth. This original voluntary standard is referred to today as the AM "audio mask".
NRSC-3, introduced in June 1990, established audio bandwidth and distortion standards for AM receivers.
All of these standards have now been accepted by the FCC as mandatory for licensed AM broadcast stations.