A Apogee buddy of ours in Jamaica runs tripple Stacked Quads and used to repair them. He did say that the standard versions of the Quads are simple very limited. He replaced the foils (1/4th the thickness), stronger magnets, different frames, tripple stacked and driven by handbuild SilverKing amps (purely silver [incl. circuit traces])....spposed to be freaking good. But in the end, its not a Quad anymore ;-)

-Flo

PS: On a side note, if you stack two speakers you do not get twice the bass, clarity, or dynamics ;-)