The midi range, not the tweeter, is the most important driver in a speaker.

No, 100% disagree.  Every mid I've heard since early 70's i hated. 
I avoided all 3 ways with mid ranges past 40 yrs. 
I am sticking with  a  MTM, allowing the lower W18 to act in low fq.s the upper mid bass meeting the Millenium tweet which is really a  true Mid range/Tweeter. = you get 2 drivers in one. 
IHow is this possible?
I just understooda   few weeks ago, after owning the Thor MTM for now 17 years..The Millenium employed the latest technoogy at that time Neo magnet. Which is XXXX times more potent vs your old regular magnet tweet. 
The upper W18 carries the low mid fq's up to meet the low FQ's of the Millenium = No need fora  old basrking/coughing/blaring midrange. 
I HATE all/every midrange. 
Millenium, the finest midtweet ever designed. 
Finest tweet every tweeted. 
paul
the honest stereo review guy

@erik_squires ..Agreed, and my RTA’s and eq’s do as well. I’m monitoring both line And the ’room’ (the latter w/a calibrated mic into a ’stand-alone’ unit). Mid-range, as approached by the bulk of the ’conventional’ speakers marketed, is where voice and instruments Are....

And, Yes, the crossover is where the crux is....*serious stare*

And, this is Why I’ve moved to a digital xover since I don’t rely nor listen to a ’one box wonder’ for my ’listening distraction’. ;)

My ’reference’ amt’s will reach down to 500hz; below that, the sub picks up @ around 150hz.

That leaves a very important gap that’s important to infill. Currently I’ve using 6" with a 6" passive, quick enough to keep pace with the amt’s. And be x-overed to not compete with the subs.

The big issue for me is a woofer that’s fast enough to keep up with ’what’s above them’. *L* We all have our ’issues’, I guess....;)

...and I’m just an ’outlier’, anyway...
...and my Walsh are an entirely different animal...as well....

But, that’s what ’memory files’ is for...*L*

(*ah* Re 12/1 @ 12:04am: One has to watch what one ’trolls’, even if only in jest....’flame wars’ start with less.... ;) *G*)

To think about it, everything else being equal, the mid probably is more important than the tweeter.  But what's more important is the xover.  The designer will ultimately be the final decider as to the final sound.

woke up in early hours and realized a  thought about the OP query. OK, Mids extremely important. Thats obvious. 90% of the muisc is in the  low mids- upper mids fq's,. Any old woffer can give low bass, and old tweet can give super highs. 
Its this critical mid fq's which confront every  speaker witha  a  challenge. So far to my ears in the past 17 years, no speaker has delivered gorgeous mid fq.s as has the SEAS Thor. I wouldn't trade my Thors for any speaker in the world. None. 
= IOW I'd put my Thors + a  basic sub vs any speaker in the world, and woop arse on dynamics/sound stage/hi fidelity, and most of all , the human voice. Now grant it, the 845 tube will surely equal/surpass the Thor in light jazz, but in complex full blown classical/opera, the 845/FR will sound anemic as it struggles to juggle all the fq's filling the tubes plates with tons of fq's. 
= 1 845 can not perform what a  dual KT88/KT90/dual tube channel can handle. 
Also I should mention the SEAS Thor  requires/demands a high performance tube amp. 
My Cayin MT35 overloaded when attempting to push the Thors. Maybea  MT45 could handle the load, but prefered the MT50 or MT55, Or cayin KT88 4 tube chassis.  all three would be acceptable. The Thor's are~~~87db~~~ so only the finest tube amps are qualified, at least 45 RMS pure class A/B power = heavy duty high quality expensive japenese trans. 
As I say, the MT45 might be able to deliver to the Thors,,My Jadis Orch REfer  had no issues pushing the Thors. Jadis makes the worlds best trans. Next best are the japenese high quality trans,. which cayin anda  few other tube amps employ. along with the required guts. many chinese tunes amps have no guts,,its all empty shell. 
Here takea  look at the Thors kit parts,,,There is no way a  cayin MT35 gonna handle a  parts inventory such as the Thor.
IOW if you ain't got a  high fi tube amp, leave the Thors alone, go buya  B&W speaker. for Class D sound,.
Class D amp /Class D speaker.

https://www.madisoundspeakerstore.com/mtm-speaker-kits/new-thor-kit-pair/

@ Andy, yes the xover is critical. 
Which is why SEAS has engineered the xover for this Thor. The xover I have in my 2002ish Thors is the UPGRADE version,,which ain't saying much based on new technology since that time. in 2020, i will have madisound build a  new xover for the Thors,,all using Mundorf Silver/Gold,,not the SG supreme which are out my budget range. 
I did manage to buy  2 10uF Gold/Silver Sprene off Ebay at 1/2 price,, so will  ship these to Madisound and add the Silver/Gold caps /new R Ressist Mundorf and new coils,,The lower caps going to the low midbass i will use a  basic tin cap/Mundorf.
The Excel drivers will blossom with sound. The dead will come tIn 2020, i plan to make about 50++ vids documenting all my new tweaks. I plan to be #1 mod guy on YT.