Let me add my voice to the chorus of high praise for the C7ES3. I originally chose that model because more than one Harbeth employee mentioned off the the record that upper management considered the C7 line the "best-sounding" of what Harbeth had to offer (whatever "best-sounding" means -- I can’t get into that here, but regardless, that was a convincing endorsement).
I’ve had my pair for, I dunno, 10 years?, and in that time, upgraded from a sub-$1K amp to a $19K T+A. In every case, yes, in terms of voicing and cohesiveness, these little boxes rivaled the Quad ESL (57s) that I’d restored & enjoyed for over 20 years. I’m sure that many people have other favorites in this price range, but overall, these non-flashy little speakers are stunningly lifelike in many ways -- at least in my 14x17 room, paired with a sub.
Having said all that, they’re not perfect. They throw a decent soundstage, but don’t disappear as well as some of the competition. Despite their generally excellent imaging, I can always point to the speakers with my eyes closed. However, given all the other strengths of the Harbeth house sound, that’s never been a showstopper for me. they still regularly provide a thrilling listening experience with content like SACD or 45RPM vinyl.
These comments only speak to the C7 stand-mount models, which are the only ones I have extensive personal experience with. But my understanding is that the main ways in which the larger and smaller models differ is in bass extension and dynamic range (and, to a minor extent, to treble extension). All Harbeths are reputed to have similar house sound.
As for the thin-wall-construction controversy, all I can say is that it seems to work. Aside from the fact that it’s nice to have 15-pound speakers that produce this level of SQ, I did personally confirm that part of the BBC’s design philosophy was to not even try to eliminate all cabinet resonances -- but to instead minimize them and then incorporate any residual resonances into the speaker’s sonic signature. That is, the overall system design expects the cabinet resonances to exist and is tuned to complement them.
I verified this by damping the cabinets externally, just out of curiousity. And when I did so, the sound degraded considerably, lifeless, dull, colored. I guess that’s one reason why Harbeth recommends minimizing coupling with even the Harbeth-recommended Sound Anchor stands.
Bottom line for me is that the Harbeth line -- especially the C7ES3 -- are undoubtedly worth considering in their respective price ranges. Not for everybody, not for every system, but still an extraordinary box speaker in its price range. Add a top-notch sub like a Perlisten, and you’ll have a heckuva good $5-10K system for a small or mid-sized room.