I think the subject of this thread is quite interesting. I think what it all comes down to is that a very hot cup of espresso like any other hot beverage or food masks the basic flavours (sweet, bitter, salt, sour). So when the espresso cooled down, the bitterness of the dark roasted beans became more pronounced making it unpleasant, whereas the increased sweetness/acidity and lower bitterness of the lighter roasted beans made the espresso more comfortable.
.... Which is why I always let my espresso sit in the cup for couple of minutes before drinking it - and therefore I don't care about prewarming the cups either. I know it contradicts some of the basic first things we learn about espresso - that the portafilter and cups should be as hot as possible so the temperature of the beverage doesn't drop too much. But if I drink an espresso right out of the machine, I find that it dulls the tongue masking the flavours - which might be a good thing if your coffee is bad to begin with. But if you have made a well-balanced quality shot, I always find that it benefits from cooling down just a bit (obviously we're not talking a huge decrease in temperature, just a few degrees)
I guess this is also the reason why professional cuppers use cooled down coffee?