I can!t tell you about the NAD receiver but warm would be more towards bass sound and can lack some detail. Bright is less bass and very detailed but can be hard to listen to for a long time. Instruments can sound shriekish and tinny especially in a very cheap cd player. You try to balance it out, if you have a warm sounding receiver you can get brighter speakers and if you have bright speakers you can get a warm sounding receiver and it will help balance the sound. You hear the terms laid back or forward. Forward would be much more dynamic sounding(lively) whereas laid back would be less dynamic sometimes dull and not lively. Forward sound can sound very good because of the dynamic result but there is plusses and minuses to everything you have to decide what is right for you. My suggestion would be to go listen somewhere where there are people with knowledge that can demonstrate the difference and let you decide what is best for you. Some of these sound differences can be very subtle and others can be quite significant. You can get too caught up in listening to hear brightness, warmth, laid back and forward. Sometimes you will see a professional review where the reviewer says he forgot about listening to the different aspects of the sound and just sat back and enjoyed the music or movie and that is the end result you want. Hope this helps.
Nad is not a warm nor a bright receiver, at least in my experience. They pair very well with almost any speaker, although it will bring out the limitations of the cheap ones.