How does solo piano help you evaluate audio gear?

Very true...

I used Moravec nocturnes Chopin or Feltsman Bach Well tempered Klavier....Only these 2...They are very different but good in their own way...The Moravec recording though is unsurpassed both musically and sonically...But the dynamic of the Feltsman recording is sometimes stunning for bass notes tuning ...

In an incremental process of tuning the room for months you must know the recording very well and it must be a good one....

IMO, the problem with using a piano as a reference (generally) is that there are so many poorly recorded piano pieces. The piano must/has to be the hardest instrument to get recorded correctly to fully reproduce its the broad and nuanced (detail) sound capabilities, much more apparent live.

You must be able to recognize these three very different timbre:

But getting a chorus right with all details is the END goal...Single voice, piano and separate instruments timbre and even orchestra are STEPS in the tuning process... Chorus are very difficult to be done right because the voices timbre could be huge in number and near one another like cello and violins are...

Here you must perceive and distinguish 8 voices....2 females and 6 males...

- higher and lower scale in frequency range than any other instrument.

Perhaps right after the organ. Bagend makes an Infrasub organ loudspeaker for 8 hz and small pipes can go beyond hearing.