Why not the piano as a reference for bass

@billstevenson 

At the risk of stating the bleeding obvious, electronic pianos contain sound systems, with all their faults and idiosyncrasies.  At least some of Yamaha's Clavinova range contain cone drivers and use piano wood for the pulp

Drawing from our experience in building both acoustic pianos and high-end speakers, Yamaha CLP-785/795GP speaker cones use pulp from the same spruce wood used to make acoustic piano sound boards. The result is a more natural, piano-like attack to the sound that is more piano-like when you play the keys.

Testing bass is not testing a deep frequency but a lot of  band range interacting...

If an audio system/room do not reproduce  well a piano you had lost your money....

To test bass i use preferably piano, tuba, and organ ..

Re piano. I rediscovered an album from long ago that I found in the cut-out bin of a long-forgotten record store. The title is "audio symphony", Check up your sounds Vol, 1. It was produced by RCA Japan in 1976 and distributed by Audio Technica.

It has a 14 page insert describing all the equipment used and even includes the music score sheets. Although it goes into minute detail of the equipment used it does not describe the piano. Notable is the use of an Ampex 440c and Studer A-80MK II reel to reel. Both running at 30 ips.

The sound quality is astoundingly good with dynamic range and quit background even on my modest system, The bass quality of the piano is remarkably good but I would agree that it is not the best test of a systems bass agility and depth.

Also, as someone else mentioned bowed string bass (in this case a trio playing in unison) is also a pretty good measure,

Could you name the artist in this album ?

Effectively  the viol is also an instrument useful to test bass rendition...

Re piano. I rediscovered an album from long ago that I found in the cut-out bin of a long-forgotten record store. The title is "audio symphony", Check up your sounds Vol, 1. It was produced by RCA Japan in 1976 and distributed by Audio Technica.

All I can tell you is what my ears tell me.  For reference in my own listening room I have recently replaced a Yamaha C7 with a Clavinova.  I did this because to my ears the difference when the C7 is in tune was indistinguishable from the Clavinova and more importantly for me, the latter is always in tune.  I am not a pianist, however, but rather a drummer for whom the piano is a means to an end.  YRMV.