Ah, so much to take in. Yes, my Velodyne has stereo high pass digital filters which is how I relieve my Krell / Quad ESL-63 / ESL-2905 of having to play low bass. It is also old enough to have a pulp/paper cone but if the servo mechanism is good enough, who cares? The only things stopping me adding more are: the cost; the space; the fragility of my home; the fragility of my partner; the neighbours and ROI.
The original Quad ESL is often known as the ... ESL. But it is totally different in concept, design and implementation to the later Quad ESLs which have factory designation not less than 63.
The original ESL, aka ESL-57, uses a curved panel (much later this was apparently "invented" by Sound labs) to give an approximation of a line-source, albeit horizontal. Like modern stadium systems, these can be stacked vertically. Stadium so-called line source systems are curved in the vertical dimension to get them to emulate a point source! It is of course rather hard to bend a flat panel in two dimensions simultaneously.
The ESL-63 and later models use fancy electrics and shaped electrodes to get a truly flat panel to emulate a point source. It makes no sense at all to stack them, nor to remove the electrics which are their unique raison d'etre in my opinion.
