Why passive bi amplifying exists???

[dlaloum]

That thing is going to be highly capacitive, so not friendly to avr amps especially the lower tier ones but as always, it depends on other factors too, for example if one never turn the volume pass say 50 dB below 0, then all may be good, and obviously -50 is just an extreme example to make a point.

I think your Quad amp may be quite suitable for the CDT because they apparently don't use a lot of negative feedback anyway, so less prone..

Incidentally, Quad also make electrostatic speakers right? So I guess they know how to deal with capacitive load lol.

Exactly! - Like many electrostatic afficionados, I long ago became aware of differences in amps, drive by capacitive loads.... Hence my having a number of amps "floating around" (!) that are all very stable into low impedances and reactive loads.

Talk to owners of Martin Logan Esl's about the struggles with amps! (I stuck with Quad ESL's right up until WAF drove me to the Gallo's)