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The Quad Electrostatic has been validly criticized on a couple of fronts, in so far as high fidelity reproduction is concerned. The first is a lack of bass, which is in fact not true. They have plenty of well-controlled, accurate bass, but you might like more level? The second is an overall inability to go very loud. This is true, with single pairs producing 85-86 dB/W/m it is not particularly efficient compared to the "head-banging" brands that make a lot of noise for a few watts input. They are however quite nice in the living room playing pleasant music. Actually, I blame the second criticism in some cases on the way people site the speaker, not caring too much about how they "aim" it. If you're off axis too much then you do experience an inordinate drop in SPL. This is true of all electrostatic loudspeakers of high accuracy. I maintain that as soon as you try to get a wider sound field, you give away some of that superior detail and accuracy.
Anyway, these problems can all be solved by stacking a pair of original Quad Electrostatics so that a tall line source is produced. This gives you, in the words of Peter Walker:
"Another thing people like to do is to use two of our panels, one above the other. This is quite reasonable because it is really a strip source, you can extend the strip source without deteriorating anything. All you do is add 6dB at the bottom and 3 dB everywhere else. It gives you a louder sound, a more impressive sound. That's all right. Adding woofers has never been very good." [ Audio Amateur, 1978 ]
I would like to add that with the proper room treatments to absorb some of the extra reflections caused by those higher SPLs, you can improve the depth and width of the sound stage nicely too.
Perhaps the all time record holder in the Quad Stacking field was Alistair Robertson-Aikman, the owner of SME. He had a stacked pair in each corner of his (massive) listening room. It is rumoured that he also experimented with up to 16 of the speakers arranged in an arc across the sound stage - money no object!
Stacking Quad Electrostatics is not a trivial task. Firstly, you have to get two pairs in good working order. Not so easy in today's world, no matter how much money you have. Then they have to be checked and maybe refurbished by an expert so that you know they are in good order. You have to have a sizeable room to play them in, or it's not as good as it can be, and so on.
I've attached some older articles on stacking Quads here, and some photographs of my own. I hope this might give some idea to prospective "Stackers" of what to expect from a stacked pair.
“Double Quad” by Dr. Tom Farrimond, Electronics Today International, 1975
“Quad ESL Mods and Stacking” by Chris Beeching