Comparing Quad ESL63 to Klipsch is like comparing very good low wattage Class-A hi-end audio amp to a mega watt P.A amp.
Yet as a pairing a low wattage Class-A amplifier calls for high efficiency speakers like Klipsch’s Heritage line, wheres the Quad’s necessitate much higher power without ever achieving live-like SPL’s (before going up into a mushroom cloud), insofar you’d rightly incorporate this important aspect into a true High Fidelity criteria with a hard deck above 95dB’s and doing so effortlessly.
Two totally different approaches to making music, discerning "audiophiles" how are not interested in bring the house down will prefer the ESL63’s, head banger’s and the partially deaf will prefer the Klipsch.
Two different approaches indeed, but an all too predictable stance from the so-called "audiophiles" to ridicule what their choice of speakers do not offer them, turning this lack into a virtue even, and then going on to "elevate" themselves to a complacent, "discerning" state of knowing-better about true sonic cultivation [eyes rolling].
With all due respect to the Quad’s, within their limits they are truly remarkable speakers - especially, I find, regarding coherency, tonality and refinement/insight, all of which are part of my own sonic preference-list. However, in my book dynamic capabilities, an effortless presentation and the sheer sense of inherent power and size are also vital aspects in setting free the music as an unencumbered and emotional event before you - an aesthetic experience, even, when the nail is truly hit on its head. One wonders whether the "partially deaf" aren’t really predominantly among those who do not realize this, but I gather it’s much more about a state of mind (and point of reference to work from) than anything else.