- just for guidance: the 2500ยดs really will i m p r o v e your sound experience beyond belief on your fine 6216 speaker system; but you should not put them on top of the speaker - but on a high spot (wall/book-shelve/ceiling). Actually, the cubes were not really needed for the 6223's - since they were equipped with dome-tweeters for high frequencies. However, even when you are lucky to have the 6223's - you will always want the cubes to make your setup complete (to look at :-) ).
If you find a pair of cubes, please remember, that they might need new units - normally, they will be outworn - and even distorting sound. These 8ohm units are hard to find - but not impossible. I am sure that Frede could assist.
Good luck!