Hi Loomis,
I think you are referring to connecting the pre-outs to both the sub and to the inputs of the amp's power amp section, rather than connecting the pre-outs to the sub and the sub's line outs to the power amp section. If that is not the case let me know, although the considerations either way are somewhat similar.
That approach would probably work reasonably well. But I suspect the resulting sonics would be no better, and perhaps slightly worse, than connecting at speaker level. The main reasons being:
(a)The input impedance of the NAD's power amp section, at least for the C356BEE version, is specified as 20K in parallel with 1 nF (1000 pF). That is a large amount of input capacitance, which the output circuits of the preamp section would have to drive in combination with the added capacitance of the cables to the sub, as well as the input impedance of the sub itself. (The sub's input impedance is unspecified, but the input impedance of the line-level inputs of powered subs is often low enough to be an issue). The 80 ohm specified output impedance of the preamp section would seem to indicate that it could handle that reasonably well, but depending on the accuracy of that spec and on how it may vary as a function of frequency I wouldn't assume that the resulting effects on the signal path to the main speakers would be negligible.
(b)I suspect that the line-level connection approach would stand a somewhat greater chance of resulting in hum or other noise issues than the speaker-level approach, although chances are it would not be a problem either way.
(c)Although some folks would disagree, a philosophy espoused by REL among others is that it is preferable for the signals provided to the sub and to the main speakers to both reflect the sonic effects of the main power amp.
Best regards,
-- Al