The only way to tell might be to remove the drivers and assess the components in a modern light. Drivers back in the tube era were often efficient but had limited excursion, small AlNiCo magnets and paper capacitors. The suspensions will have stiffened, glues failed, caps dried out, cloth surrounds become porous, cones can become brittle and crack. Not to mention abuse.
Aside from the basic limitations when compared to today's drivers, there's likely several of these mechanical or electrical issues that should be addressed before one seriously compares a truly vintage speaker to contemporary standards. It could be the speakers being auditioned had multiple problems.
OTOH, even though many of the old speakers may well have been state of the art for their day, that was then. There's very few 40+ year-old designs that have stood the test of time. If you like the cabinets, it might be fun to upgrade the drivers and crossovers. That worked for me with a pair of old Fisher speakers.
-Ed