most advanced speaker in a rectangular wooden box?

As you move up the scale of ingenuity and price, speakers take on different, sometimes fantastic, shapes and are made out of different, sometimes exotic, materials.  So what would you nominate as the most advanced design that still houses its drivers in an essentially rectangular box made of some kind of wood product?
British brands : Spendor, Harbeth, Graham, Stirling Broadcast, Audio Note. These manufacturers realized long ago that there’s nothing wrong with wide-baffle monkey coffins if implemented correctly. IMO, many of them can go toe-to-toe with the exotic stuff from Magico, Vivid Audio and the like. This is especially the case when it comes to recreating the natural timbre of stringed instruments. They also tend to be less fatiguing.

Some can’t comprehend how this is possible because they focus on measurements and marketing jargon. Instead they should rely on their ears. The proof is in the listening.
Oops you said “Rectangular”.

British brands especially Harbeth and big Tannoy’s.

+1 @helomech.  Yes, so mainly Brits: ATC, Harbeth, Spendor, Tannoy, and let's not forget PMC and Proac.  Top of all those lines are some great speakers.  Apparently more than one way to skin a rabbit.
Harbeth speakers are musical, with a very good sound field, and look good in a traditional setting. I am very, very happy with my Compact 7s. And I have listened to a lot, starting with McIntosh speakers about 50 years ago. You cannot go wrong with products from this manufacturer.
The key word is "Advanced"  
Not a lot of truly new technologies...  
Look at passive crossovers... Off the top of my head, I can think of maybe  8 types of slope technologies besides the amount of slope applied,  yet today, there are mainly 3 types with varied slopes of technologies used. There are more used of course between Parallel and series circuits, but still most use 1 of 3.  All 3 have been around for alooooong time. 
I have seen new driver technologies, with materials used and how those materials affect the overall sound. 
Back in the late 70s, we were playing with active speakers, there are advancements for sure.  Harbeth and Spendor are well very well done. The Avid speakers mentioned are sprinkled full of truly advanced technologies, but of course, they are not in a rectangular wooden box. 
In the end, what makes a great speaker are not Yet Advanced Technology.  Don't get me wrong, even old school technology has improved... what, how?  Driver tolerances, capacitor tolerances, resistors be 1 to 5 percent.  All of these things have improved what we hear regardless if it is an old school paper cone or a titanium, beryllium or carbon cone. Tons of improvements even in the "Not so Advanced" Camp.   

I'd vote for something vintage - The Yamaha NS1000 - vapor - deposited beryllium dome mid and tweet in the 70's?!!! Pretty advanced - there are manufacturers now that don't quite have beryllium figured out.

If one is looking for what are *genuinely* the most technologically advanced speakers, ones without the flashy gewgaws and questionable designs often found in consumer speakers, I suggest one search among professional studio monitors. They are generally no nonsense products designed by professionals for professionals, and the technology is in the research, design, materials and electronics of the monitors. The more expensive monitors, especially, are bullet-proof, resolutely clean, coherent, transparent, almost ruler flat across their frequency range and offer pinpoint imaging. They usually aren’t fancy to look at, but are made to be absolutely true to the program material being played. Sonically, they will almost always disappear, and one will find one’s self "listening down the wire" to the source rather than the speaker.

Many of the studio monitor brands do not sell to, support, or waste money advertising to, the consumer market. Thus, brands such as Amphion or Quested are unknown to most consumers. The big exception is Focal which has a very respected line of pro monitors, although again, one won’t see their pro series monitors advertised or reviewed in the likes of Stereophile. Nor is one going to be able to wile away the hours in some plush salon auditioning any number of them. Pro monitors are mostly sold by dealer and colleague recommendations, auditions within one’s own studio or the studios of others, and reviews in pro magazines or on websites like Gearslutz. Thus, less of their price is spent on promotion and markups and more goes into the making of a genuinely excellent speaker. In short, one gets significantly more bang for the buck.

My personal experience and recommendation would be monitors by PSI Audio, which are designed and built in Switzerland. (PSI is an offshoot of Revox. Does THAT ring a bell? The Revox quality shines through.) They are expensive, but they are all "active" speakers so one needn’t be concerned with or pay for amplification. And PSI monitors are as technologically advanced as any fully analog speaker made. No DSP trickery. They should work VERY well in an average sized room, even the small ones, and I would presume that they would offer an excellent WAF as well, especially the A-215M. See:
Zu Definition 4. . . . or any other speaker with a built-in, powered subwoofer.
Perhaps perceived as too old fashioned for some, I've been pleasantly surprised at how astonishingly good my recently acquired Klpisch Heresy IIIs sound with a pair of subs. Updated here and there, but they get the "Been Around a While" award for model longevity. They require  well sorted amps and cables to get to a "high end" level, but man…a great speaker.
The Heresy certainly get the award for most square inches of driver you can put in the front baffle of a speaker.   
They require  well sorted amps and cables to get to a "high end" level, but man…a great speaker.
wolf_garcia:  What amp(s) and cable(s) are you using with your Heresy III's?
Schiit Freya preamp with Sylvania "chrome domes" and Tung Sols, Dennis Had Firebottle HO SEP amp with KT120s, Amperex 6SN7GTB, and a Shuguang 274B rectifier, Morrow and AQ ICs and AQ Rocket biwire speaker cables…and a Schiit Loki between the pre and amp (usually bypassed but there when I need it). REL Q150e and 108MKII subs.
Infinity 2000A's with RTR electrostatic tweeters! I have a pair bought from the original owner - a California college professor. Nice!
@roberjerman, oh how I lusted for a pair of the 2000A's in 1971! Their only competition was the ESS Transtatic, which also used the same, great RTR ESL tweeters---three of them. I have a pair I found in 1982, though they haven't been hooked up in years. While the 2000A had a normal looking enclosure hiding it's unique guts, the Transtatic was a 42" tall structure, with overhanging top and bottom blocks of walnut-veneered MDF.
Wolf is dead on about the Heresy really singing with quality electronics...I’m running a CJ Classic 2SE pre and a pair of Quicksilver Mid Monos and my Heresy III sound awesome with 40 watts of EL34 goodness.  
I'd like to see Klipsch tuning the Heresy box with a port (likely requiring a different bass driver), and putting the mid and treble horns on top of the box as it would provide sort of a steam punk vibe…of course, then it wouldn't be a Heresy anymore although perhaps more heretical. That said, and having said that, my Heresy IIIs are great the way they are. Oddiofyl is 100% correct, has superior taste, and is likely kind to animals. However, why he's a mad power freak is mysterious…40 watts? INSANE….
Dynaudio states the Special 40's are the most advanced monitor they have ever made..They sound like it IMO...
I'd say the JBL M2.  You do need the special-sauce Crown DSP amps (or a BSS processor with the appropriate EQ curves), but with the special waveguide and the DSP it has an almost perfectly flat frequency response throughout its range not only on-axis, but a near-perfect off-axis response as well.  
They are all fighting the laws of winners here only losers who advertise. 
^Any speaker is fighting the laws of physics. I've heard monkey coffins that can make some planars sound like a distorted mess.
Another vote for Proacs, although the original Rogers LS3/5a circa 1976 we quite a nice little box.
It’s funny that many of the answers in this thread (but not necessarily unique to this thread) are just people posting speakers they like instead of answering the actual question from the poster. In this thread the OP’s question was "most advanced" and "rectangular"/box, as in interesting materials, construction techniques, new driver arrangements, driver types etc., not "most pleasing, most natural, warmest" classic design.

Instead I see people nominating classic speaker which while fantastic and classic and landmark, by definition are not the most advanced standmounts in a square box today. If I asked what’s the most advanced car today that is still gasoline powered, you wouldn’t say a Porsche 959 or McLaren F1 even though they were landmarks at the time and still eminently collectible valued and esteemed.

I think evolutions of driver materials and types (particularly tweeters), bass porting, time alignment/driver arrangement/phase tweaking, cabinet construction and now re-thinking the traditional two wires going into a passive box are genuinely changing what you can expect at every price point. ATC showed what you can do with active speakers a few decades ago (even though it really rubs audio tweakers the wrong way in some aspects) and now the LS50W and Dynaudio Xeo show me we still have a long way to go in that sort of evolution. I would love to see a cost no-object active standmount from the world’s best leading edge designers, so that the trickle down and market awareness can grow.

If a speaker has inferior sound, regardless of materials/design, it's not advanced in my book. The whole purpose of advancing speaker design is better sound. If speakers containing beryllium tweeters, carbon fiber woofers and built-in amps sound no better than a plywood box and paper cone, then they're not more advanced, they're a failed attempt.

From all my speaker auditions, including Magicos and the like, I concluded that speaker advancement hit a wall about two decades ago. Some newer designs can be more revealing or dynamic, but they totally fail in creating realistic tone or timbre, or they're fatiguing to the point that a 30 minute listen results in a headache. How is that more advanced?