Hi Raul, it is difficult to define such a subjective term. I agree with you about the location of recording microphone and actual listening position in a concert hall and all that. On the other hand, I am not sure about the difference between warm sounding stero equipment and instrument/hall. Let see if I get this right. Most people tends to imply that roll off high frequency and the bump in mid upper bass seems to give rise to what we hear as warmth in stereo equipment. Now, acoustically dry concert hall, as I understand it is a hall the have very small reverberation time, say 2-3 ms where as the acoustically more pleasing tone hall, those that seems to be richer, denser tonal color, usually has longer reverberation time, around 4-6 ms (I read about this somewhere a long time ago, not sure if I remember the number correctly). However, richer, denser tone does not neccessarily means warmer sound. Depending on where you sit in the audience, there is definitely going to be some roll off of high frequency in comparison to close mike position. Sitting at 10-30-50m away from the stage, one would expect certain decline or roll off of high frequency, more so than low frequency by various instruments to exist. I think that's why real symphonic concert seems to sound less bright and duller than what I hear on recording and that's why orchestra nowaday tries to compensate for that by raising the frequency on the note to give that extra brightness. Whether the roll off characteristic of each hall give rise to warmer sound in some concert halls over the other, I am not sure.
When talking about different made of the same instruments, warmth is definitely a term that I often hear people using to describe the different in sound, French vs German tone, brands of pianos, various wood used in instruments. However, I have not heard anyone specifically say that it is the characterisitic high frequency roll off of mid/upper bass bump that give rise to the warmth quality in each instrument or not. Personally I play piano and own a Grotrian grand piano. I have also played quite a bit on Steinway, Fazioli, Yamaha, Ibach, vintage Erard and Pleyel and I what perceived as warmth tone when compare each instrument is definitely quite valid.