In 1929 Wente and Thuras designed the Western Electric 555 speaker driver capable of 100-5000 hz freq. range with an efficiency of 25% (compared to 1% of todays drivers). This was “compulsary” as only very low powers (<10watts) were available. They had to fill huge 2000 seat cinemas with sound with very little power….necessity is the mother of invention.
The 555 is a great driver…I mean really great, see the frequency sweep below strong from 70hz upwards to 6000hz with no loading. I have had Goto, the 570dx, and it gets nowhere near that. With Gotos you always fear someting is going to break. 555 have been runing for many decades….suffered abuse of the working enviroment…they are still around with original diaphragms!
Using 555’s is one hell of a comitement to sound. You need big and long horns (15a/b, 16a/b….), power supplies; not for the casual hifier at all.