I believe that the reason for this phenomenon is the non-linearity of human hearing.

Greg.

Another factor may be that when we play an instrument solo, we play it louder than we hear it when listening to music with the instrument in the mix, and this reduced volume of the instrument in the mix results in a brighter sound. (I think I read this somewhere, plus that's what I observe as well)

Greg.

It'll be interesting to experiment... too bad I haven't any suitable project for backing CP-80 lined up at the moment!

Greg.

I wonder if the big rock stars of the era possibly made a point of having stiffer hammers in their stage pianos to get some extra punch?

...I agree that the current CP-80 add-on is quite mellow - as was the original instrument that we recorded.

FWIW, when I play my own sampled version of a CP80 it sounds roughly the same, and also I do remember the real instrument sounding like that too. (been a very long time since I've played a real one though)  Aside from all the processing they do for recordings, another factor may be that we often listen to the radio through small speakers, which cut out the bass and lower mid-range. Just a thought anyhow.

I'm just a home hacker and lack a reference grade setup. I seem to always prefer the sound of my HD570 headphones, but I have read that these are unnaturally bright.  It doesn't matter how much I boost the highs - I can't make another set of headphones (HD455) or my loudspeakers (cheap hi-fi bookshelf speakers plus a subwoofer) sound as good as the HD570s, but the sound through these headphones may be quite different to most folks equipment.  Anyhow, when I emphasise the highs of the Pianoteq CP80, and listen through the HD570s, the sound is very clear and very metallic - just the way I like it.

Greg.

I'm just curious about the virtual restoration. You tried to mathematically compensate the lack of brightness from the old strings. Why not just change the strings for new ones?  Was the specific Yamaha strings for this model no longer available?

Changing a real set of strings requires:
- transport the piano to the workshop
- remove the strings
- order new wounded strings (by a string maker) or make them if you have the spinning machine for that (don't know the english word - "tour a filer les cordes" in french)
- mount the new strings with new tuning pins
- tune a few times
- transport  the piano back to the customer
- retune a few times
I let you guess the time involved and the price...

Virtual restoration: about one hour working with the parameters, no other expenses

as stated by Greg here and Pethu in another thread, I agree that the current CP-80 add-on is quite mellow - as was the original instrument that we recorded. We now have added a third preset to the CP80.ptq file, called "CP-80 restored bright", where the mezzo hammer hardness has been increased from 0.48 to 0.64. Of course other changes can be done by using the various parameters...
Philippe

Very good work Philippe! 
   Thanks for this new beautiful instrument.

    I'm just curious about the virtual restoration. You tried to mathematically compensate the lack of brightness from the old strings.
    Why not just change the strings for new ones?  Was the specific Yamaha strings for this model no longer available?

     Or maybe it was just a nice mathematical challenge to work, virtual revive of old strings, I supose

     I have a sugestion. Some people are used with the controls of equalizer like the rolling buttons from the real CP-80, for bass, middle and trebble.   Sowhy not add the option in the top of equalizer scream?  The rolling buttons would alterate the equalizer curve.

    Take a look in this fast representation I did: 

Greg.