Thanks Duke. As an engineer, my principle when approaching audio is
1. Everything in audio engineering is compromise, sometimes technical, sometimes financial, sometimes both. So nothing is one size fit all.
2. Before I can form any opinion about technology, I must audition the best or the most technically accomplished implementation of that technology in their most suitable working environment. If not, the opinion is misleading and worthless.
3. If there are scienctific papers about this subject, the approach is not just reading the headline of this research and repeating like a mindless parrot. The testing condition as well as any limitation of the reasearch need to be understood. The most important thing is that opinions, even from trustable experts, which are only the consequence based on result of the test, not the direct statistical result of the test, need to be evaluated with critical thinking rather than treating it like a Bible.
So I spent nearly ten years trying to hear as much as speaker as possible, especially the ones which measurement has been published, to ensure the quality of tech implementation. At that time, I don't have resources to own expensive speakers, my speaker was a pair of modest KH120A. But thanks to my connection, I have chances to audition them in suitable environments. here is my experience:
1. "normal" speakers: I can't count how many of them I have heard, B&W801, Revel Salon2, ATC SCM50, Neumann KH420, KEF Reference 5, Harbeth SHL5, YG Acoustics Carmel, Sonus Faber Stradivari, Thiel CS3.7 are the most notable ones. Out of those, the jack-off-all-trade is a Revel Salon2 and KH420. But the one I like best and want to own is Thiel CS3.7, too bad they are out-of-business after the death of Jim Thiel.
Even though I like some "normal" speakers, but the more I explore them, the less satisfy about them I get. No amount of placing them can result in a soundstage and imaging I like. Put them near side wall and I get a big but very shallow soundstage, plus the very blurring image. Put them far from side wall, ok I get good imaging now but at the same time soundstage is restricted. It is a big problem for well-done box speakers. Other speakers tries to overcome that by letting peak in frequency response, but it is only sound good with a subset of music recordings. And I don't want to be restricted.
2. For omnidirectional speaker, I have heard the quasi-omni speakers like Bose 901 long before but I don't like them due to their messy FR. I also do not impress with MBL 101 speaker at the show case. Then one of my friend bought a 101D and I got a chance to audition properly. The moment "Aha" happens when we pull it out with the distance from walls is 2 meter. Soundstage is immersive, imaging is great as well, absolutely sumblime. The downside is that the estate to accomondate them. 2m out of side wall to each speaker means that one dimension of room need at least 6m. Due to the fact that the listener need to be 2m from wall as well (to avoid LIBR) then the other side of room will be 7m. So a room 6 m x 7 m is needed. My friend's room is 50m2 but it is not something I can afford just for listening space. The good implementation is rare and expensve as well.
3. Dipole speakers: I heard Magnapan and even owned the LXmini, but while they exhibit some special quality in soundstage, their shortcoming in terms of high frequency extension and low frequency volume is too much to bear. But at least they are promising, so I go to build LX521.4, and it clicks. Small baffle lead to more even off-axis FR, bass module with 2x 10" high excursion L26R04Y per channel is decently enough. Side wall 0.5m-1m with toe-in and 1.5m from front wall is enough to get the wide and deep soundstage with good imaging as well. Another good thing is that it requires room only with around 22-25 m2 to work well, with dimension like 4m x 6m, which is suitable for me.
Even then, LX521.4 is a compromise, albeit a good compromise in term of cost. Living with it for a round six month, I began to notice its shortcoming. It has nothing to do with the concept or its execution, but rather on the cost constraint of its. The MU10RB driver has an uneven FR in working range (2k-2.5kHz) lead to a little bit rough in upper midrange around 2kHz. Secondly, both U22REX/P-SL and MU10RB has high breakup at 500Hz and 1.5kHz respectively which create the not so detail sound signature. Thirdly, their dipole pattern brakes up at 2.5 kHz due to magnet blocking in MU10RB and only comebacks at 6kHz, when dipole tweeters take more responsibility, so it is not a ultimate implementation of dipole as I hope. After that time I go all in with dipole by diy with nude-driver configuration (which I learn from CharlieLaub) and high quality driver like SS 26W/4867, Purify PTT6.5M and Mundorf AMT 25D1.1 plus bass module from LX521 and two nearfield ripole subs. And for my room, it is until now the best for strictly stereo sounds.
4. Cardioid speakers: I only heard the D&D 8C. They are great speaker aside from a less than effortless low midrange, which somewhat remind me of LX521, so I suspect it is because of an 8" Seas driver they use. The nicest thing with great implementation cardioid speakers like 8C is that they can be placed near front wall without detrimenting sound quality. So we need only to focus on the side wall, and a distance of 1.5m-2m from side wall is not that hard in most normal room to create the wide soundtage and great imaging. The depth of soundstage can be done by small toe in but not as impressive as dipole and omni speakers. I think that due to lack of rear dispersion, cardioid speaker is more suitable for multichannel setup, especially the center speaker, than dipole and omni speakers without the need of big room. That why I am in the process of building floorstanding cardioid speakers dedicated for multichannel sound systems.
5. Horn Speaker: In my opinion, great horn systems, with horn supported frequency down to 100 Hz and the right recordings which cues from hall is embedded properly in, is the most lifelike sound system I have heard. Problem is that these recordings are very rare, like great 1960s stereo like Mercury, RCA Living Stereo and Chesky. Modern reverb cue is artifical and sound weird to my ear. Due to the horn, it is even more comfortable in terms of placing than cardioid speaker. But the downsize is the huge size for horn supported frequency down to 100 Hz lead me to thing that it is not for me, just like omni speakers. Most smaller horn systems with horn supported frequency down to 500 Hz, while still sound very dynamic, have an abrupt directivity change which don't like.