NAD's traditional sound signature is referred to as 'neutral' by their fans and by most professional audio reviewers. There is another term called 'flat' which is used generically in the audio world. The definition of 'neutral' and 'flat' with reference to this context, essentially is the same. It is good to have an understanding of what this means or refers to before using this word as a reference point for NAD amplifier purchase or use.
Frequency response is a measurement of a device’s magnitude and phase output in response to an input source. It is usually measured within the range of human hearing, typically 20Hz to 20kHz. Any amplifier or speaker exhibiting a flat or neutral response is more accurately reproducing an input source, through to the output without enhancements in a particular area (or range of frequencies). In other words, a flat or neutral response means what comes in goes out at louder volume. The flatter or more neutral the response, the more pure the audio. Most fans of NAD equipment are chasing this 'purity' when they refer to this 'neutral' or 'flat' frequency response. As a buyer or user of NAD equipment, it is important to understand this philosophy. For example, those who desire listening pleasure over purity may not like NAD equipment.
Terms like 'warm', 'smooth' etc are highly relative, subjective and are based on perceptions of the buyer or user (or reviewer). It all comes down to personal preferences. For example if a person who enjoys listening to bright and sparkling sounds, listens to a so-called 'warm' amplifier, he can also review it as being 'dull' or 'lazy' or 'boring'. The other way round, if a purist listens to these sparkling sounds, he may refer to it as 'bright' and 'harsh'. The genre of music also has a role to play and so does the listening tastes of the prospective buyer and user. It is important to select or use audio equipment that sounds good to you, rather than to someone else, or in the laboratory
To answer your question specifically, NAD amplifiers have a reputation of being able to play well with most speaker types, makes and models. Hence the pain of mixing and matching speakers with NAD amplifiers, is easily avoided. However again, you may want to consider your listening tastes (genres of music loved and frequency response preferences), your listening room topography, the overall application and your equipment preferences.
All the very best with your search.
Afterthought: What is the definition of a purist? There are many definitions but to sum it up, all in one - a purist is perhaps a listener who expects to listen to his/her recorded music in as much, the same way he/she would have listened to it, sitting in front of or among the performers
