When coupled to a matching impedance system (smiley face EQ) the 604 is historically nominally flat from Fs - > 20 kHz measured in free space, though in room with all its variables can be even wider, though of course not nearly as flat.

You can get a 'taste' of this with any wide/full range driver/speaker system by inserting a suitably sized/spec'd pot in series with it and find the widest/flattest response in-room; which I used a cheap one to dial in stereo systems, sometimes to the point of tuning the cab's vent(s) individually.