The 600's are special speakers with the Aerolam cabinets that were later used on the 700's. You do need to be careful with the drivers since they are no longer available, and they were also the last Celestions to use the copper dome tweeter, like the original sl6. That tweeter is less extended and efficient than the aluminum dome, and by now I'm sure it is well tarnished.
Those are special speakers, but they will only really open up and show you their best with good solid power.