The main thing you need to be concerned about in older speakers is the condition of the drivers - dry rot of foam surrounds, deteriorating of cones & domes and such. Dahlquist, DCM, Design Acoustics, Quad, B&O and many other good brands.
I’m not too sure of the assertion that good speakers only come from that era. There are plenty of worthy speakers which are newer, and still being produced today. Paradigm, B&W, KEF, Tannoy, Def Tech…the list goes on and on.