The issue is the room, EQ and levels. Fix that and you can have a mind-blowing ESL experience. :)
Also, fixing that is sometimes very hard! :)
Is tricky though. I never solved that equation back in the day with my original Quads, but that was before 1990. I didn’t bother with the Quad 63. (I do like the bass on the original Quad and room positioning can make a difference- what’s there is taut and tone-full). I used to have them on ARcicI stands which really reduced bass but made them ear height-- I know people still like to get them up a bit but one answer is to sit lower. :) The little feet that are stock give them a proximity to the floor that helps the low end.
With the horns, I found that adding an additional set of woofers, beyond those integrated into the Duo, helped. Adjust position, crossover point, phase and I cheat and use DSP on the external subs. Still took some time to get it right though.
The Finnish company Gradient made OB/Dipole subwoofers for both the QUAD 57’s and 63’s. Though the subs used dual dynamic cone woofers, they being employed in OB/Dipole fashion allowed them to blend better with the QUAD’s (because of the inherent advantages of OB’s, and the dipole characteristics shared between speaker and sub) than any sealed, ported, or infinite baffle sub.
Ad a reminder (I have spoken of them here before), Rythmik Audio and GR Research designers Brian Ding and Danny Richie have collaborated on a joint product, their OB/Dipole Servo-Feedback Subwoofer. A very unique sub (or even woofer---it can be used up to 300Hz), THE sub for QUAD’s, and any other dipole speaker, whether ESL, magnetic-planar, or ribbon. Or even dynamic---Siegfried Linkwitz used OB/Dipole subs in his loudspeaker kits, very similar to the Rythmik/GR (and almost as good ;-) . Whart, you should really look into the Rythmik/GR for use with your QUADS, and even your horns!