Walk-in soundstage

@pindac  Thank you for introducing Q Sound. You must refer to The Happiest Days of Our Lives on The Wall by Pink Floyd for the helicopter spatial sound effect. Q Sound was subsequently used in, for example, The Division Bell, to achieve 3D effects. Additionally, to name a few, Roger Waters’ Amused to Death also incorporates this technique.

I’ve noticed that the “walk-in” spatial sound effect becomes more prominent in my system with the latest recordings that contain these spatial cues, even when using traditionally designed (box-type) speakers. In that setup, the elevated spatial height achieved with my Harmony Micro DAC becomes an important factor I would say.

I would also like to revisit the unanswered question I posted earlier regarding the significant roll-off above 10 kHz of the Quad ESL panel. I believe this characteristic is one of the sound traits that warranted his stacked HQD systems implemented by Mark Levinson, where:

D: Decca Kelly Ribbon (> 7 kHz)
Q: Quad ESL 57 panel (100 Hz – 7 kHz)
H: Hartley 24" woofer (< 100 Hz)

Again, I am curious what experience the end-users like yourself have and how you cope with this.

https://www.stereophile.com/content/mark-levinson-hqd-loudspeaker-system

@lanx0003 I hope I'm on the same path as your inquiry as a reply to your query.

I have always had the 57's as a 40Hzish to 17Khz listen and accepted them for this.

Most recorded music falls within this frequency range, so the 57 user is not sold too short on the entirety of the recorded music experience.

I can say with total confidence there are other Speaker Designs that cost up 20-30 x 57's using average UK prices as the guide. Where designs of Speakers with a cost of £15K - £25k can't offer what the 57's can do with certain recordings in the upper mid's and maximum frequency. At the same time a lot of monies can be spent to create a End Sound that surpasses the overall 57 experience, but is noticeable for not being as attractive in certain areas of the produced End Sound.          

Stacking 57's supplies a 3dB increase in the Upper Frequencies and a 6dB increase in the lowest frequency and what is said to be a improved flatness to the frequency response.

Adding a Subwoofer is a failure in the demo's received for both 57's and other Quad ESL's. Myself detects the change in the sonic of the Bass and this becomes a detractor as the presence of a sonic trait that differs is detected. 

As I use a range of Speaker Designs I am not off the view the 57's or other Quad ESL's are lacking in Bass, the models are delivering the least adulterated Bass, which becomes very very attractive when lived with. Experiences of other Speaker Designs, some £20K+, are able to produce Bass that can become an unattractive experience as the shortcomings are recognised whilst exposed to their End Sound. 

If a Upper frequency driver is able to be used that blends seamlessly, then why not, I am yet to hear a seamless blend driver used in conjunction with a Quad ESL.

I have ideas, own the drivers to be used as part of the design, but have not rushed to create the experience.

When a End Sound is wanted to be heard different I simply swap out the 57's to another Speaker that creates a Bass that is much more familiar to the average Speaker user and has a extension of the Upper Frequency.

The ESL has been a very good teacher, very few Speaker Designs remain in the system that have noticeable Colouration, it is knowing the ESL very well that allows such a prompt decision to be made about an alternative design of Speaker.

The the Irony, I have to have a poor designed Speaker in waiting, the use of such a Speaker fits perfect with my recreating experiences of PA Produced End Sound, from a time when a young man was out and about having social experiences inclusive of live music experiences. Early era Chicago Blues also sounds wonderful when the End Sound is noticeably coloured from a Poor design Speaker.

Halcyon Days with great memories. 

@pindac  Adding a Subwoofer is a failure in the demo's received for both 57's and other Quad ESL's. Myself detects the change in the sonic of the Bass and this becomes a detractor as the presence of a sonic trait that differs is detected. 

I wasn’t surprised that the sub fails to integrate well with ESLs, given how difficult it already is to achieve good integration with traditional box speakers. I’m actually not too concerned about bass performance with more modern ESLs, such as the 2805, since they can reach below 30 Hz.

As for the treble, I’ve been considering adding super tweeters, such as ribbons or AMTs from brands like Aperion. They offer crossovers starting from 8khz, as well as adjustable attenuation levels to help address sensitivity differences. However, issues like phase coherence, synergy, and sonic character could still pose challenges. You might be right that ESL enthusiasts often accept—and even enjoy—the speakers as they are, without worrying much about frequency response. I was amazed by how realistic and natural the sound from an electrostatic panel can be.