The CLX is a transparent window in the recording with precise dynamics, but I would call it a "transparent box speaker ", in some aspects approaching the Magico Q7. Great exact presence, but does not fill the room with music as the Soundlab's do, and is not as correct as the ESL 63. I must confess that due to its very low impedance in the high frequencies, I have listened to them always with SS.
BTW, all opinions based on listening mainly to acoustical, non amplified music!
We all have our preferences micro, and therefore for my part, I would not quite agree with your considerations of the three main transducers that you reference. I too have many years of running various ESL's including every model of Quad up too and including 2905's and in my own opinion the later models are more complete in the way they render music across the audio spectrum than the 63's, and in my pretty extensive period of assessing another speaker in comparison with the Quad I found the CLX to be superior to my incumbant 2905's in all areas, with the exception of sub 50 Hz , the Quads subsequently traded in against my current CLX Anniversaries.
You are quite correct in your comment regarding the rather tough load that they can present to amplification and I have run with several makes, models and topologies. Ultimately I found the CLX's to perform at their best ( thus far) with regard to midrange tonality, soundstage etc, etc, and with room filling presence you mention, required +150 Watts of good valve amplification, to that end I am currently running with ARC Reference 250SE's.
Whist my experience of SoundLabs is far less comprehencive by comparison to the other brands, for my taste, I found the SLabs whilst comfortable and easy to listen through, exhibited to me a slightly unnatural texture and tonality in the mid and upper range, as compared with CLX matched with an amplifier that maximises its strengths and does not struggle to drive the speaker correctly, as the latter scenario may result in a lean sounding presentation.
Again merely my own experience and consideration.