Mark, I've been experimenting a bit with the placement of my Catalysts and filter settings on my pre-pro.
From your description with the coaxials at ear-level there is a 2dB peak around 5500Hz. Many people who use these speakers in a clasical stereo setup (not behind an AT screen) will have the coaxials at ear level.
One of my colleagues owns several pairs of Quad ESL-63's (pro- and regular versions). All are 20+ years old and in very good shape. Needless to say they sounded awesome! (for stereo)
We listened to a lot of setups, even with ionophones. What amazed me is that these loudspeakers reveal staggering amounts of detail while dropping off 2db at around 5kHz (!!) and linearly drop off further to -6dB at 20kHz.
The standard Catalyst has a much more flat frequency response, and depending on loudspeaker placement a 2dB boost (!!) at 5,5kHz. After some experimentation with filters I noticed that this boost is very noticable as 'brightness' and with certain recordings can easily become fatiguing.
With my TAG I tried to emulate the frequency response of the Quad's using "High Shelf" filters at 5kHz (-2dB), 8kHz (-1dB), 11kHz (-1dB), 14kHz (-1dB), 17kHz (-1dB) and 20kHz (-1dB). This adjustment made the Catalysts sound a lot more forgiving, although maybe a bit too much for less bright recordings. Also, I'm pretty sure, the phase response is degraded by using this extra EQ.
The Catalyst amp has 2 DSP settings, so (for me) ideally it could contain one setting for the intended placement behind an AT screen and another one where it is placed in a more stereo minded setup. Maybe with a little less steep drop off and some reduction of the 2dB boost at 5,5kHz.
Mark, is something like this feasible?