I looked for ESL 63s for a number of years on the used market...my experience was that when one speaker had been repaired and the other not, or two speakers had been repaired at different times...the speakers sounded different from each other. I don't know if this is an issue with Quads today...I finally bought one of the last new pairs of US Monitors sold in the US. Check with QS&D...they are knowledgeable, experienced, and reliable.
Good Luck!
Good Luck!