I do believe that if one amp/preamp sounds substantially different when level matched operating within their linear region, then one of them is broken (possibly by design).
In my opinion, I don't believe one design is broken if it does not sound the same as with another design. Speaking out of common sense, all designs, be it amps or speakers are all different. The parts inside the amps, the type of parts, the arrangement of electronic boards etc. are all different and that explains to why they sound different. In general/theory, an amp is supposed to be neutral, in fact everything should be designed close to as neutral possible so as to reproduce the signals in their true state. This doesn't appear to be true in actual case. Designer's may be striving to achieve neutrality but most equipment more or less have their own kind of sound due to the points as mentioned above. The term is subjective and debatable. In fact, I don't know how we can classify any particular equipment as neutral. Only if we can measure neutrality.
More technical persons would be able to explain or clarify in better terms.
In response to the poster of this thread, it would be good to get a quality integrated amp as opposed to using your NAD as a line stage in the long run.