Car audio home audio ?

Well for what you are doing, I'd recomend an MTM design.  The Extremis 6 has some crazy excursion, and uses our XBL^2 motor so you know it will be very clean excursion.  Proof of this is in the distortion measurements.  In a ported enclosure they should do just fine playing from about 4k all the way down to around 50 Hz (a larger enclosure you should be able to drop then down even further without a problem).  The 4's do pretty good down low, but the 6's will destroy them.  And honestly I don't see a need to do a 3-way set.  It will greatly complicate the crossover design and really isn't needed IMO.  As for tweeters, I'm not sure what the tbd is.  My guess is it is a Tang Band tweeter? I was actually thinking one of their tweeters would work great for you.  Here's a link: http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=264-822 

One of our dealers was comparing his Kit 81's to some $2k Paradigm speakers.  He actually did an A/B comparison in the store (he knew the owner) and the owner told him that he would be crazy to choose the Paradigms over the 81's.  What's nice is that the 81's are a great deal less money.  Going DIY is a great way to save money. 

What I suggest doing is comparing our Kit 81 design to our Kit 81C (center channel) and see the differences in response, crossover design, etc.  Hopefully that will help you in your design.  Also if you are near the Seattle area, you should stop by and audition some of our speakers as well.  We have some good engineers here that should be able to give you some pointers in your design.

Steven Kephart

Adire Audio

Wow, this is a great discussion.  I agree with most of what's been said already.

Yes, in HT, the center channel is IMO the most important.  A good MTM design seems to be in vogue right now, although for the past ten years I've used a simple 2-way I made from a 7" midbass and a good silk dome tweet.  I am almost finished with the replacemnt, which is an MTM design.

I believe the biggest mistake people make when beginning on the speaker building hoby is to skrimp on the crossover.  Yes, a pre-fab crossover can do a nice job, but using a $14 pre-fab off eBay after spending $350 on your drivers and 24 hours building a nice enclosure is like ripping the V8 out of a Corvette and sticking in a Scion 4-banger.  Hey, it will go, and it looks nice, but it will fail miserably to meet its potential.  I have often spent more on the crossover components for a system than I spent on everything else put together.  Don't skrimp - and don't be afraid to experiment.  The MTM center channel I mentioned above is using 2 $250 Revelator mid-bass drivers and a $150 Morel tweeter, and the crossover components are costing about $300 (all Solen.)

On the other hand, I put toegther a system for a friend and spent a total of less than $100 on four woofers, three tweeters and all crossovers buying special sale items off Parts Express (Tang Band tweeters and Vifa woofers and pre-fab 2-way Dayton Xovers.)  We made 2-way mains and an MTM center channel.  He built the cabinets and was very happy with the results.

About the drawbacks of that ribbon tweeter.... Umm.. the price? ;)

Ribbon tweeters are a big fad right now.  They are very pricey, but can sound very good.  I just don't kow if they are worth the cost.  From what I understand they don't offer any real advantage.  You can probably get regular tweeters that sound just as good for cheaper.

Also, I wouldn't suggest dropping the tweeter down too low.  If you do, you will limit the output level you can run them at.  That's kind of why I suggested an MTM design so you can get some low extention with authority without early breakup.  The Extremis drivers are good up to 4k, so that should open your tweeter choices up a little bit. 

Unless you want to spend a great deal of money on amplifiers for each speaker, I don't think active is the way to go for the crossover.  I'd recomend sticking to at least polypropolene capacitors and air core inductors and you should be fine.  Just be sure to watch the way you mount the inductors.  You want them to be at right angles from each other to prevent interference unless they are mounted a good distance apart.  Here's a tech paper on that: http://www.adireaudio.com/Files/TechPapers/Crosstalk.pdf 

Steven Kephart

Adire Audio

I'm not familiar with that Fountek ribbon.  I have used THIS one recently with fantastic results, and it can be safely crossed as low as 600Hz although I'd recomend no less than 1200 or so for power handling reasons.  My favorite ribbons are the Heil Air Motion Transformers... hard to find these days and kind of pricey at around $600, but the best sounding high end going IMO.  Carver, Ravens and Infinity make some excellent ribbons.

Ribbons have a couple of advantages over dome or cone transducers in that they have wider frequency reaponse and better horizontal dispersion making them less colored off-axis, although they lack vertical dispersion much beyond their length so speaker placement is more critical.  It's also why most designs with ribbons use multiple drivers - making the ribbon longer increases vertical dispersion.  They tend to be more efficient, although they also distort more easily at the limits of their power handling.  They are not well suited for near-field monitoring use.  Ribbons are about twice as expensive to manufacture as conventional tweeters, as I recall...

The sound of ribbons is different from that of conventional tweeters.  Most people describe it as more open and airy.

Conventional wisdom, however, will tell you to use dome tweeters for most average listening uses because you get much better overall performance for the price.  If you want to try some of the best conventional tweeters available, try a Scan Speak 9800 or 9900 (metal or soft dome versions of the same motor) or a Seas Millenium.

Stereo powered subs can work great in large rooms.  Adding them into the tower is, as you say, an interesting design... but it can limit your room flexibility in several important ways.   To extend the bass response of an MTM you could try a transmission line tower with the 6's similar to the Thor.

As far as the specific questions you have asked above concerning Adire drivers, I'll let Steven tackle them except to say that all else being equal, a single Shiva should outperform dual Koda 8's as a subwoofer.

I will comment more on the tweeter situation.  Morel makes excelent tweets,  highly recomended for overall quality.  I love them.   But do not try a top mount with an MTM desaign. D'Appolito's work best with dome or cone tweeters placed equidistant between the woofers and on either the same physical axis or the same acoustical time-aligned axis.  Top mounting makes this next to impossible.  If you are using two woofers and a top-mount tweeter, it is no longer an MTM design it is simply a design with two wooferes.  I also cannot recomend ribbons in general with an MTM design.   Ribbons in an MTM would be too limited in soundfield coverage as you'd have to use only one driver.  Ribbons are best in a 2-way configuration with two to three drivers stacked on the same vertical axis for best room coverage.

Bottom line, actually, is to go for it.  Try different things.  If you hate the results, you're only out time and a little MDF as you can build new cabinets or modify the ones you make.  Dirvers are always movable from one enclosure to another (your should see my garage littered with picked-over carcasses of failed enclosures!)