I find this review very interesting because it demonstrates how the most common objective measurement - frequency response - does not tell the whole story!
These days, there are many many speakers that measure with flat frequency response, both on and off axis, but the subjective listening detailed in this review makes it clear that similarly flat frequency response does not mean that two different speakers will sound the same.
It does seem as though a near-complete lack of distortion plays a big role in the clarity and fidelity of the sound though. Less expensive - or more accurately, less well-engineered - drivers (the tweeters and woofers) may offer flat frequency response, but higher distortion, and this could account for a large portion of the audible difference in clarity.
Naturally, one wonders if it is possible to have this same level of clarity, but at a lower price! It is certainly nice to know what is possible in terms of sound quality, but for many people, $6000/pair will not be affordable.
Personally, I have no issue with very limited output below 80Hz. I am a believer in using a subwoofer with ALL speakers and utilizing the 80Hz cross-over with 4th order slopes. For my tastes, too many reviewers focus on speakers' bass response and imply that a speaker that can play lower is somehow better or, at least, a better value. I, personally, see very little value in a speaker being able to produce bass frequencies that, due to room interactions, will only muddy the final sound and would be much better handled by handing them off to a pair of capable subwoofers.
To finalize, if these Snell XL speakers are able to noticably outperform speakers such as the RBH TK and Salk SongTowers in terms of clarity and distortion-free output, one wonders what it would take to match the Snell's level of quality, but at a more affordable price. I am keen to know how the Status Acoustics (RBH) Decimo bookshelf speakers would compare as they are similarly lauded for their clarity and detail, but come in at just under half the price!