In the 1950’s all amplifiers were tube, and many loudspeakers 16 ohm (nominally). The move to solid state amps and lower impedance loudspeakers at the same time (mid-to-late 60’s) was not a matter of coincidence.
One reason the QUAD ESL sounds better with tube amps is because of it’s insane modulus of impedance. Nominally 16 ohm, but up to 32 ohms at low frequencies, down to 1 ohm at high. Used with a typical tube amp (with their relatively high output impedance, and resultant frequency response deviations), the ESL is missing high frequencies, used with solid state missing bass.