Most noticable differences are the use of steel instead of aluminium for support and handles on sides. Power handling is about 100-150W. Each speaker weights about 16kg. I was not able to locate the sensitivity for the speaker on any documentation but estimate it to be about 78dB or79dB - losely based upon the volume dial to achieve approximate volume from my SL6S.
The Pro-63 are not easy speakers to drive. Long ago I owned a Pioneer A400X which was able to drive the SL6S effortlessly. However when I transferred the speaker cables from the SL6S to the PRO63, there was no sound when the volume was at 9 o'clock??? Initially I though either the PRO63 or the A400X had developed an issue. I decided to slowly turn the A400X volume until I could hear something. Music was finally audioble at about 2 o'clock!!!
The Quad ESL(s) are well-known for superb and realistic imaging but poor bass. I normally use the Sound Dynamics THS-1000 subwoofer (100W) to accompany the Pro-63 (and SL6S).
My Pro-63 are placed on the non-Quad stands which were design with a gently tilt to project the image upward.
I was lucky enough to obtained the pair from a well-off gentleman who had several pairs of Pro-63 in his home. The serial number is 29xxx and estimated to be from about 1988.