I'd like to respond to the original post not to disagree with it, but add my experience with two completely different approaches to excellent speaker imaging. I've used (and am a dealer for) Dutch 8C and all the Linkwitz designs, including the 521.4. So my comparison is dipole vs. cardioid.
The Dutch 8C, when toed in, sound like what you expect when mixing a recording: pinpoint accuracy in a soundfield defined by the speaker side-to-side limit, with depth cues those from the original recording. Live pop and well-recorded classical retain a deep, convincing soundstage from whatever vantage the recording engineer intended, whether up front or some distance back. Ordinary pop recordings with emphasized vocals put the singer very far forward but stable. In all recordings, but particularly well-engineered ones, the buttery-smoothness of the frequency response above, say, 150hz or 200hz is just pleasurable beyond belief. That is even true with the speakers near the front boundary because the design controls for this. I believe this carefully-controlled dispersion, which aims to minimize early reflections, is a very successful approach, and it is better than any marketed speaker I have ever heard. I have not heard the Genelecs -- I'm trying to become a dealer, but product is so scarce that Genelec is delaying taking on new dealers. Finally, the tweeter of the Dutch is incredible -- triangles sound amazing.
1. The Dutch deep bass sounds like speakers and does not have the low distortion of a dedicated subwoofer. Ideally, you roll them off 2d-order at 45-50hz and let a devoted pick up from there.
2. The Dutch activate room modes like ordinary speakers. Correction can only go so far. My current setup has a fearful cancellation at 63hz, for example. I can tame the reinforcing modes, but they never go away.
But now take a diametrically opposed approach, the Linkwitz 521.4 dipole. The principle, which I find interesting and insightful, is that by allowing room interaction, but with a significant delay, the secondary and tertiary reflected sound causes the brain to, in essence, correct for delayed sound and construct in the listener's mind the correct soundstage as if the speaker had the carefully-controlled dispersion of the cardioid. (That is what I think is not generally appreciated about the design principle of these speakers.) But with this important difference from a cardioid: the reverberant soundfield is all there. The practical effect, for whatever reason, is to make the soundfield recede quite a bit, and vocals, even in ordinary pop recordings, are farther back. The soundstage is broad and deep, and the speakers are difficult to perceive as distinct sound sources. The sound is utterly beguiling, with a wide-open feeling. Part of the latter is that there are zero box resonances. Part of it is also that open-baffle woofers have a more natural sound. This is not what the recording engineer intended; it's what the design accomplishes to improve 2-channel recording by manipulating sound in a certain way. Siegfried was adamant about that point.
The drawbacks of the Linkwitz dipoles are significant:
1. You must have lots of boundary distance for the speakers, and the ideal version of the speaker's placement also requires a very distant rear wall, which I suspect very few people can have in an ordinary home.
2. You have to learn how to set up a complicated 8-channel-amplified setup with an external crossover. No matter how many times I set these up, it's always tricky -- and I've been involved in audio since 1978!
3. By no stretch does the Linkwitz have the buttery-smooth response of the new breed of active speakers like the Dutch, Genelec, Neumann, and KEF. I don't know why. (And yes, you critics, it could be my build, so please don't feel the need to say as much.) You can fuss with the crossover and EQ parameters all day, but unless you're an engineer, I doubt you'll get the response linearity a company with an R&D department can produce.
4. In reasonably-large rooms, open-baffle woofers cannot pressurize the space to achieve true subwoofer levels. The drivers flop like fish but can't produce high SPL below about 35 or 40 hz. I have always used a powerful sub in a corner to pick up the bottom end. This makes the system that much more complicated. But I like recordings with super-deep stuff, plus I use the speakers in a video setup.
5. In a video setup, the recessed vocals make dialog harder to discern.
Which do I pick? Linkwitz, at the end of the day, but only because I can deal with the drawbacks to make it work. For most people, most of the time, in most ordinary rooms, the Dutch 8C (or their equivalent in Genelec, Neumann, KEF) are wonderful.