It is common sense. the sound is split in lows, mids and highs. All of them can not be forward, because it is measured relative to one another. If highs are raised compared to lows and mids, it means that lows and mids are recessed compared to highs. So if all three are forward, it means that all three are also recessed. Can you see that does not add up?
This isn't a fact for me either, I really don't know where you got this from but I don't agree.
well the lows, mids and highs are not just one freq each, its a range of frequencies. Each range can have a peak on its own. So yes It does add up. Pull up any proper equalizer and see for yourself
+1!