A tube amp sounds darker if you put a higher load right? Same for SS amps?

[Helmholtz]

Don't forget that NFB equalizes changes in the frequency response (in the linear range of the power stage), that could be caused by a different load impedance.
Voltage NFB counteracts changes of output voltage and current NFB counteracts changes of output current. In result the frequency response will stay more or less the same.

[qweiczz]

FAR more likely it is the result of the different speaker rather than the impedance-ness.