Since the Dc9 shares the same design, I would guess that was the case for this great aircraft to.
Bu to day I came a cross a NW Dc9-14 flightdeck picture.
Photo © Jean
As you can se, a headset (to me it looks like a David Clark) is provided. From my experience this headset is far bigger, heavyer, and more noisecancelling, than the smal modern "phone" headsets.
Any reasons why they use that headset in the Dc9-14. Is it used in other Dc9 too?
My best guess must be the -14 beeing shorter and thus more noisy up front.