Technical question to MAGNEPANs Owners ?

Is it true that this kind of speakers can degrade the sound with a sudden increase in ambient tempearture? Is it also true that they must be used within strict frecuency band limits? Have any of you had these or other problems with Magnepans? Thank you.
I've recently picked up a pair of used MG IIB's. They had sat for quite awhile unused. I fired them up as soon as they were delivered. The sound was good but not great. After a day or 2 and several hours of playing, things really improved. I don't know if it was the ribbons adjusting to the ambient temperature or that they needed some burn in from sitting idle. (they actually sound very good for 15-20 year old speakers) I keep the temp round 72 during the day and set back to 68 at night. I don't notice a difference in the performace.
Let me add that the freq response is very good down to around 50 hz for sure. Very solid using a 35w push/pull class A tube amp.