I have always used well recorded piano compositions as an analytical tool for determining system. I will attempt to explain how I use piano recordings as an analytical tool. Rather than concertos and symphonies, it is best sonatas. I have some sonatas that, rather than solo piano, add a violin. That is in my opinion the best analytical tool. So how and why? …
Crossover and Driver Integration: The piano, as others stated, is a singular instrument covering a broad frequency range. Problems with speaker driver integration and crossover design, or issues with ragged frequency response of electronics become easily evident by using well recorded piano recordings. As an example, in my past, I had hybrid Apogees. The lower piano registers were slow and smeared compared to the wonderful midrange and highs.
Dynamics: While not the best test for percussive crescendos or rock dynamics, the piano by definition as a percussion instrument, is one of the best instruments to use for analyzing system dynamics, especially microdynamics. While I agree in part with @audiokinesis that the fundamental and first overtones do not have the same impact as other instruments, well recorded closed miked piano will have strikingly vivid first wave percussive impact. The real value is in analyzing the microdynamics of secondary harmonics and how ppp notes are developed in a composition.
Detail Retrieval: Well recorded piano is an excellent test for detail retrieval including, harmonic decay, the sound of the hammers vs the strings vs sound board, as well as how the notes decay into the venue. Piano recordings can easily discern engineering techniques such as close miking vs far miking. A well designed audiophile system should be able to discern these attributes of well recorded piano. Here, combined with my discussion on dynamics, is where the piano is excellent for bass analysis. Analyze bass note first wave (fundamental impact). Is it sharp and striking. Analyze how it decays and the sound of all the secondary harmonics. Determine if you can discern the hammer strike from the string sound and sound board. This is an excellent test for comparing equipment.
Imaging: Closed miked piano recordings will be able to develop an image that shows the position of the mike relative to the keyboard. Image palpability will be evident.
In conclusion, piano recordings are an excellent analytical tool in general, and for bass as well. Yes, you will need to use other records of other instruments to analyze bass impact, but these others are not as good for bass detail and especially for bass integration.
Apologies for my usual pontifications and typos in advance.