What speakers can b considered as world reference?

Speakers that are perhaps most Live-like are the type used as studio main monitors to impress clients (musicians). These systems generally have a large sweetspot and have huge dynamic range with low distortion and are listened to from a far field position; a band can hear themselves immediately after recording before mixing/compressing all the live music for consumer consumption in the mix/mastering process. They are usually way to big to fit in a home. Generally these systems are often custom installed in walls in specially designed control rooms by acoustic experts (speaker design and crossovers may be uniquely tailored to each setup). These systems are not necessarily used for mixing or monitoring, as this is often done on smaller speakers with narrow dispersion in a near field position. Their main purpose is to impress musicians with a live-like sound, often at live music levels (very loud). If the artist is impressed they will be happy their A&R man booked the right studio. If they are really impressed then the artist may buy similar speakers and install them at home. Most consumers are not familiar enough with live sound to select speakers that would qualify as reference and most cannot dedicate a room to only this purpose. Top studios get constant use with many people and those with a good reputation are well known in the right circles and booked up well in advance. An underperforming or badly setup studio will usually get a make-over until they get it right or go out of business although some depend on their exotic getaway location more than the gear (bands love to party & holiday too). A tough world where customers are often the judge but many of the top studios have it => Reference Live sound and there are many good pro designs to choose from.

Acoustically large enclosures in the tweeters' pass-band make diffraction a problem which is exacerbated by 90 degree enclosure edges.

Agreed. This is such an obvious and well documented drawback of any speaker that it is amazing that so little is ever done about it by audiophiles, as it is simple to correct. In prestigious studios the main monitors are usually built in walls and this setup completely corrects for the above issues and others to boot!

Since MBL seems to depart from most other designs as far as being omni, what are people's opinions of MBL's?

I think Omni or dipoles are similar. They will load the room more evenly and you can achieve a natural sound with cheaper drivers in a dipole....but too much reverberation can be bad and if you place these close to a rear wall then it may ruin the imaging or even risk sounding claustrophobic or cluttered. Like panels these designs should be brought well out into a room and will work best in a large room. Dipoles work great in movie theatres....especially for surrounds.

He said his major design constraint when engineering the Bose cubes was how to get decent sound pressure for a $4 manufacturing cost. $4!!! How did you think they got that huge advertising budget?

I believe it. Sadly, even on high end speakers the engineering budget is miniscule. Wood work and veneer is most of the cost. Most of the really high costs go into a better looking piece of furniture, you know, aesthetics, industrial design, rather than acoustics. Although you would never know this from the advertizing spin.