Yes that is some what my point. If there is a difference in the formats it will be at the extreme highend of the frequency. Not sure many of us really (myself included) here it anyway.
I guess my point is the difference is very small, and our systems (and ears) have much bigger issues.
I know my system does not put out anything above 17khz. With Sennhizer 650 headphones I hear to 19 but on my speakers it is dead after 17khz.
Why is the only difference in formats evident in the high end extremes? I've never heard that before. What are you basing that statement on?
The difference between your speakers and headphones, beyond the obvious limitations of the specific transducers, is one of isolation. Your speakers abilities are profoundly affected by the room, the contents, your seating position, etc. Those factors have no effect on headphones, which have a much greater degree of isolation. That's probably why they don't use speakers to test your hearing.
BTW Sennheiser HD650's are not known for their upper-end extension. In that region between 10-20khz they drop down severely just after 10khz averaging around -15db! At 19khz they are -10db.