Hi bobcox,
In what is a very complex technical matter (too long for discussion here), think of solid-state amplifiers has having essentially 0 ohms output impedance--the speaker "sees" an impedance of about 0 ohms at the other end of the cables. So the amp's output does not interact in way that changes frequency response of the system, as long as the measured frequency response of the amplifier is ruler-flat, and virtually all are, unless they were designed by a bonehead.
However, this is NOT the case with tube gear. Tube amps have output transformers with fixed impedance taps at 8, 16 and 4 ohms. This may interact with the speakers impedance curve and introduce response anomalies. The latter often account for some of the mystique surrounding tube amps-- subjective descriptions of traits (depth) attributed to tube gear. Accuracy is not a virtue of tube amplification.
Regards,
