What are the smallest speakers that are clean and flat down to 20hz?

Gotta be the single-panel Maggies, Wolf. But the old 3-panel Tympanis have plenty of bass, and of a very high quality There are people using the two bass panels of Tympanis as woofers, and even as subs!

QUAD ESL’s have even less bass than Maggies, but are not nearly as big.

Wow tostado, a piano tuner! Man, that takes a real good ear and lots of training. I worked with one pianist who wasn’t much of a player (he grew up in the era of the British Prog Bands, hence couldn’t play rhythmically to save his life) but was a competent tuner, which is what he ended up doing as a career. Surprisingly, a lot of real good guitarists had a hard time tuning before the invention of the electronic tuner. They would get each individual string close, but chords would still sound sour. The only guy I played with (R.I.P.) who was great at tuning a guitar had perfect pitch, and playing with him spoiled me for most others. Pianos, with two strings per key and many more keys than the six strings of a standard guitar, are much, much harder to tune. Twelve-string guitars are notoriously impossible to keep tuned for very long, especially the thin-necked Rickenbackers. Jim McGuinn was always out of tune on live Byrds shows!

But yeah, real low frequencies don’t sound like a "note" being played. And a lot of criticism of subwoofers sounding fat, bloated, slow, etc., is more the result of "room boom" (dimension-related resonances/standing waves), the very low frequencies the sub(s)are playing exciting the room modes/nodes that aren’t excited without the sub(s). One of the benefits of dipole woofers and subs is that they excite fewer of those room dimensions (the length not the width, if firing down the length of the room), the result being less room boom. Harry Pearson long preferred the bass panels of the Magneplanar Tympani loudspeakers (a dipole, of course) for their very taut, lean bass and midbass, in preference to omnipole dynamic cone woofers in enclosures. Some fanatics still use the Tympani panels as woofers/subs, and GR Research in conjunction with Rythmik Audio offers an Open Baffle/Dipole sub that uses a pair of 12" woofers mounted in an "H-frame". State-of-the-Art bass!

The lowest frequencies produced by almost all musical instruments are not as low as commonly believed. For instance, whilst 28Hz was above stated as being produced by an upright bass, that instrument's lowest note---the E string played "open", is actually located at 42Hz. The 4-string electric bass (often incorrectly called a bass guitar, even by John Atkinson, who should know better) produces that same note and frequency as it's lowest.

There are a few instruments which produce frequencies lower than 42Hz, including pipe organ, grand piano, and contra bassoon. Some recordings made in very large spaces---cathedrals, concert halls---contain the very low frequencies those rooms support. The ability to reproduce those very low frequency "room sounds" is one of the benefits of good subwoofers. A good recording of a pipe organ played in a cathedral will contain the "shuddering" sound that combination produces. It is an enormous, massive, thrilling sound!