Yes, I'll grant you the six-string bass has lower frequencies. But the majority of bassists--rock or pop or classical players-- play the standard 4-string bass, which is why I didn't get into the non-standard models.
On the subject of the grand piano, there is a special model, the Imperial Concert Grand, made by Bosendorfer, an Austrian builder now owned by Yamaha, which has additional keys added in the lower bass, so that the lowest note is around 14 or 16 Hz. It has 97 keys, rather than the standard 88 keys. The piano is not commonly used in concert or for recordings, and the fundamentals of the lowest notes are weak-- so the 2nd harmonics (28 Hz, 32 Hz) are stronger and any speaker capable of some response to those frequencies gives a convincing account of the lowest notes.
Alan
