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DESIGN GOAL: make no compromise loudspeaker that is very simple- no multiple drivers, plain and normal 3-way, easy load for the tubed amps, simple crossover.
REALIZATION:
Project 6 is "very simple":
Take a good tweeter : Raven 2
take a good midrange: ATC SM75-150Super
take a good 15 " woofer: ATC SB 100-375 SC
blend them together in a nice 120 liter box.
Make a simplistic XO using good parts.
Add silver wiring for taste.
You are in business with a 95 dB loudspeaker that is silky smooth,  and kicks ass at the same time.

They produce very unique sound. Well, maybe don't try it and go directly to Project 10.
Project 10, which is a major improvement over P-6, re-defines what is possible to make at home as a diy-er.
These speakers for me beat every affordable loudspeaker I have heard before.   

Design parameters - first attempt:

System response
System XO
Box alignment

Boy I was wrong ! The ballpark values did sound merely OK. Not worth the money at all. Mediocre sound, when I look at it now.
After rethinking it, I decided SOMETHING MUST BE WRONG. I was sure that ATC dome can do better.
So I listened to it without XO - wide open. Whoopee. What a change. Music had life and beauty again!
I understood that I must throw away all the common wisdom and measurements and PC simulations.
Back to intuition and ear judgment. Plus some ear help from good friends.
Three nights later I came up with a schematics that on paper looks all wrong, measures very well, (+/- 2 db) and sounds heavenly.
I am very proud of the XO that is a result of that job. It used to be a keeper for months . Also the impedance is a very easy load, and phase does not "turn" too much.
Comments about the XO are HERE.
The box will stay SEALED. The bass reflex destroys the fidelity. IMHO It is a problem solver for small enclosures and cheap drivers.
If I want QUALITY, I must go sealed and lightly stuffed. I estimate the box must be about 3 times the size the PC suggests for that bass driver. (for some alignments the Boxcalc suggested 29 liters, for other 47 liters - bwahahaha) Ok so sue me.

BUDGET:
Total cost: approx. 3000 USD for everything. Please remember the box is temporary. A beautiful 120 liter box may cost a fortune.

Review updated December 15 2002 and concern P-10
I feel very uncomfortable writing this. I can imagine that for many readers it will not be credible, cause you heard too much praise for about every product that exists commercially. But this is NOT a commercial product. I have no reason to lie to you.

So, are those the best speakers in the world?
All things being equal, probably YES. Not counting active speakers. Not counting speakers that go on your head like Stax Omega and Lambda headphones.
But counting everything that I heard during last 10 years in this hobby - yes! And I heard Grande Utopia and Wilson Watt/Puppies, and Audio Physics and Audio Notes, Lowthers, Proacs 5's, I mean everything I possibly could. And to my ears no speakers share so many virtues in one, relatively simple, plain package. Easy to build, not expensive in Hi End terms, not obtrusive visually, and above all infinitely musical.
Do they have best this and best that etc.? I dunno. They are a lucky strike, a combination of features that blends together so well, that as a designer I cant believe my luck. The sound is completely free from categorization. No more evaluation of bass and mids and trebles separately.
Let me tell you the most outstanding features:
1. disappearing act - unbelievable
2. recording insight - fantastic, sometimes scary. You know what and how was recorded.
3. teleportation to the original event - with goose bumps
4. sheer impact from just 1 watt (at 95,5 db/W/m) fan-tas-tic. Fast, especially unexpected transients impress the living daylights out of people. I love to scare them.
5. Amplifier friendliness- easy load, but mercilessly reveals playback chain flaws. Forget about transistors, or badly matched electronics. It would be a waste of time.
I can imagine, that with all SET amps, with all  good amps period, like with Cary 805, BAT VK60, Lamm, Audionotes, Wavelengths, etc. it could rock and sing.
6. The purity of highs - at first I was skeptical. Then I put Jensens in and I sat back in disbelief. Such is the airiness, purity and resolution of treble that it opens a whole new world of information, previously smeared. You can hear things that happen on the mixing console. You can back-track the editing actions. You can count tracks of the multitrack. Still, the "musical" aspect is absolutely beautiful. Not a trace of sharpness like from focal inverted dome. Nothing like that.
7. The mids are not real - I mean WITHOUT a back chamber how can it get so pure ?  ATC tell me !!!  I can sit and listen to all my fav records until bright morning and love every bit of it.
8. Bass is MUUCH better defined with sealed enclosure. I am rebuilding the present 117 liter box to a super strong and sophisticated 80 liter one. And think of all that time I wasted making Scan Speak bass. Until now I cant adjust to this ass kicking, fast, powerful bass presentation. The 8565 Scan speak sounds positively broken in comparison. I hate to criticize what I loved just one month ago, but I had no idea what can be achieved.
9. Those speakers are quite  easy to build. On drawings below you can just see. Any box of 80-120 l will do. No chamber for mids. Can be done in one day with proper tools. Costs not too much depending on the finish.
10. The XO is VERY SENSITIVE to changes. Depart from my design and forget it. Do not attempt to use any caps other than premium. Just go Audionote or Jensen PIO and do not waste time for experiments.

Downsides:
None for me really.
The only sure thing that characterizes their sound is the total lack of own character - everything sounds different. Every CD is different, and if the CD is a result of many different sessions, you can tell immediately.

The mid dome is very very transparent and neutral. The Raven enters at 5 kHz . It just helps the dome to extend the top end.
Metal sounds positively metallic, but the voices DO NOT sound metallic. Cymbals sound just sooo large and free, they have the zziing and sizzle, but the sizzle is ON THE RECORDING, it is not an artifact like with cheap metal tweeters (JBL titanium springs to mind). The speakers are very efficient, but without any downsides of horns. The detail is placed in front of you in such a natural way, that it is the opposite of the artificial detail that some speakers THROW at you by lifting the upper end. The mids have no boxy coloration, that is so damn difficult to eliminate from cones in the box.. This is the most difficult aspect of speaker building for me - getting the mids free of boxy, honky sound. No cupped hands whatsoever.
The index finger and thumb clicking sound like a human fingers clicking over the human skin, not like a plastic clicker. The applause - a one thousand hands clapping sounds absolutely credible. Not a sizzling beefsteak on a frying pan. (if you know what I mean).
The sounds are a little larger than I am used to, and the stereo picture is much larger than the rectangle between the speakers. On most recordings the "screen" of the film I am "watching with my ears" is about 4 m high and 8 m wide - almost double dimensions of my room wall against which the speakers stand.
ANY bass that I heard from "normal" loudspeakers sounded in comparison  "small" and slow, restrained in its freedom. 

You are right. Those drivers are extravagantly expensive. But on the other hand the manufacturers do most of the job for you. No need to make the box for the mids, no need to make bass reflex port, no need to worry about this and that. The challenge is to "understand" the nature of those drivers and to allow them to play freely, without forcing them to do what YOU want and expect from them. You do not want a sports car to pull the trailer so to speak.

Now I remember the last time I was equally impressed with immensely transparent and beautifully musical speakers - these were Sonus Faber Guarnieri with some VERY expensive and large active sub woofers driven by GRYPHON amplifiers and wired with THE REVELATION VdHuls.

Do I think that YOU my reader would like them ? I am sure you would.

Some construction memories:

Right tools make half of the job !!!
Please note my new router, a mighty 800 W Hitachi
 

Reading before making is not optional !
 
 

The bass driver from the inside. 8 6 mm bolts are held by the t-nuts.
 

The front plate with mid and Raven cutouts. Impossible without a good router.
 
 

The front baffle with all cutouts.
Caspar circle router plate is a must !
 

Measuring setup, with my own impedance measuring box, a sine generator,
Radio Shack lovely digital SPL meter (laboratory calibrated) with its output
connected to the oscilloscope and a digital voltmeter.
 

A test drive of the bass unit. All computer simulations proved to be just perfect.
Even if bass reflex compared to sealed version sounds inferior.
 
 

Project 6 against Project 5.