When you consider the variables introduced by the room, speakers and source (I take it we are talking yrs back and the source was analogue), the differences between a Yamaha and a NAD were probably not as easy to determine as you suggest. As to you correctly guessing it, there are numerous explanations that can be proposed: a) NAD was popular among frugal audiophiles and you subconsciously picked up that this person cared about audio, but also cared about cost; b) you clearly liked the sound, NAD was popular so there was a chance that it would be NAD; c) someone may have suggested it or you overheard it but you did not consciously process it, but it surfaced when you had to guess (especially since you like NAD); d) the speakers you saw were frequently sold with NAD...I could continue speculating, but you get the gist of it, your guess has little scientific value, it is just a nice narrative that helped validate certain experiences and beliefs.
PS, please note that the above does not argue you could not differentiate both amps in a controlled test, it merely provides a number a valid alternative as to why your guess was correct. However, the concerns stated regarding the variables introduced by speakers, room and source, are reasonable considerations to question your conclusion that you could easily identify NAD equipment the described context. An unfamiliar environment (in the context of you testing NAD) in conjunction with the other varying unknown variables (i.e. speakers that you may know, but were not familiar with in that environment, the sonic signature of the source, etc.) will introduce far greater sonic differences than most competently designed amps of the period might display (especially in a noisy environment). The "house" sound (assuming there is one) you identify as differentiating the two brands is much smaller than the sound signatures introduced by the other variables.
