The Arros should be a good match for that amp and your room size. If you get a much bassier speaker, it could overload such a small room. Since the room is small and the Arro has a rear-mounted port, you should get pretty good back wall reinforcement to help strengthen and extend the bass. Ordinarily I'd recommend more than 40 watts, but it's a small room and the NAD has good current delivery, which is important to dynamics and bass extension.
For most things these little columns should do just fine, but for movies you *may* want to supplement the Arros with a small powered subwoofer.
And don't worry if you have no preamp or sub output. I've found that many small powered subs sound just fine (and sometimes better) using the speaker connection instead of interconnect. Connect both your Arros and the sub directly to the NAD's speaker terminals and set the sub's crossover frequency to around 40-50 Hz. I've found that connecting both directly to the speakers offers a smoother, more seamless presentation *and* better imaging than running the L-R speakers through a high-pass filter.
As Pierre Sprey of
Mapleshade records says:
For seamless subwoofer sound, use only the speaker cable input, not the RCA input. In addition, connect the two main speakers directly to the main amp output, not to the subwoofer's output. .... Set the crossover at the lowest possible frequency that doesn't leave a bass gap. You'll be amazed at the overall transparency you gain.