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

@ 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. 

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/

@ MikeLavigne Great post, summing things up ~~w/o bias, no distortion~~~
I was not aware of the fq range numbers for voice/cello/piano/other important instrumentation, particular to classical muisc. 
For light classical (- nota  orch in full bloom) most speakers can handle the fq's voicing. Again , some may  prefer speaker A's warmer sound, = B&W othersa  sharper more clinical sound = SEAS especially their top design The Excel series. 
You pinned down the critical fq phase, for all my classical, especially voice, 40-250,,and i guess this is why SEAS designed the MTM, which offers 2 midwoofers ddoing the job in this range = offering a  fuller soundstage. 
I always wondered how to incorporate the 20-40 hz's, now with a  new pre amps avaliable  having to pre outs, i can employ a sub amp/woofer. 
So ideally the Thor is the best speaker for voicing a  full orchestra. 
The  1 thing to keep in mind with the Thor, is one needs a amp taht can handle the low sensitivty. @ 87db. ,,no wait, its 89db, yet 4 ohms, = requiring/demanding a  Class A amplifier, Specs say 50-400 watts, You obviously do not need 400 watss, as that high wattage will shed the surrounds. 
Yet 50 watts, will have to be Class A 50, not Class A/B, as my cayin MT35, rated A/B/32 watts ultralinear proved no match for the Thors and the trans quicjkly over heated. 
My guess the cayin MT45/50/55 might just barely make the grade to drive the Thor. The Jadis orch Refer had no issues at all. and the speaker responded with full bloom,,,I never tried other tubes, than the stock EI's, which have a  good reputation, but as  trejla has mentioned, the EI's may bea  bit over rtaed, and in this i agree. They hada   top end *steely* harshness at times,,but the voice/soundsatge was likea  real live show in your living room with jazz. I wish I had rolled the KT90's to KT88's and others, before i sold it. 
Truly a  Class A/b amp, The trans very  high quality. 
The 2 bias controls/treble/bass, near worthless. 
Best amp for the thors ona  budget is the Cayin KT88. Or Muzishare or Line magnetic, all 3 offering powerful trans, all near Class A energy. 
The thors need a  class A power, A/B  power is sacrificing /debilitating the quality of the speakers complex components. 
Years ago, there were few class A push pull amps that were afforadble, Now with the 3 main chinese lines of amps, its all within  reach of a reasonable budget. 
This is how i arrived at  the term, Class A speaker. 
Class A cd player. If you want Class A sound, you can not have a  Class A amp and the other components less than.
A
A/B
A/b
B
C
D 
F
 You need to figure out where each of your components fall. 
You can takea  Class \A/b component and make it pure Class A, but employing mods, which i am in the process of doing. 

My Thors with 2001 xover parts is really a  class A drivers with class A xover design, but parts, are now Class A/b or even B rated, since latest tech developmernts have been offered, = Silver, even Gold/Oil. 
Thus the excel drivers are pure Class A, but with xover parts being Class B, lowers , debilitates the Excel drivers to a  Class A/b performance. By updating the xover parts, the Excel drivers can now become true Class A voicing. 

Hope some of this clarifies the issues we are all facing in our quest to acheive pure true, no fake Class A fidelity in our systems performance/muisc reproduction. 


http://www.seas.no/index.php?option=com_content&view=article&id=382:thor-seas-diy-kits&catid=66&Itemid=365

Here is a Q for the board, 
The SEAS Trym is a 2 way, 87 db, yet rated a  better 8 ohms. 
Here is my Q.
Will a  845 amp work under the load of a  low sensitive speaker ~~rated 87 db~~~ yet 8 ohms...IOW will the tube have issues with the impedance of 87 db?  Maybe not full  orch, but as in  chamber, which i have a  good collection of. 
I may be interested in a  2nd system and want to hear the magic of the 845. 
My guess is the voicing of the 40-250 fq's will be anemic/weak. 

I just cked the specs,,says highly recommended 50 pure class A watts, = 845 is not recommended. ....
I want the Trym, but which tube amp to deliver the goods?


http://www.seas.no/index.php?option=com_content&view=article&id=384:trym-seas-diy-kits&catid=66:seas-diy-kits&Itemid=365

 

Hey Andy, I do not see a  listing for the Nextel 5.5 mid,,I do see a  listing fora  SEAS Nextel midtweet, does have the neo magnet of its bigger brother, the Millenium,,the Nextel tweet hasa  higher 92db efficeny,,the Millenium is very low at 89 or 87, can't recall. 
SEAS hasa   new  nano cone W22, which is built on new dewsign materials,,,Its quite expensive , but if you could have Madisound build youa  2 way with the SEAS W22 Graphene + Millenium with Mundorf Silver Oil caps (or whatever cap you can afford, IMHO, that might bea  superior way to go vs the Thors. 
This way, you might not needa  sub, as the W22 will pick up the low fq's very nicely and the Millenium will meet the W22 effectively. 
Basically you are getting the Trym , but witha  superior W22.
Instead of rebuilding my Thor xovers,,I may put that cash in the W22 Graphene+ Millenium. Have to chat with madisound's tech guys, see what their opinion is ,,they havea  great friendly straightup crew ready to assist. 
If you geta  quote on the design, please pass it on.
paul
huge SEAS fan down here in new orleans

Just cking Madisounds web,,no listing for the W22 Graphene,,,so had to google,,now i know why, the W22 Graphene was just released last month, 
Price will be $600+ I am sure, as the W19 Grap is $529. 
She looks a  dandy,,, 


https://audioxpress.com/news/seas-announces-excel-graphene-w22-8-inch-woofer-launch

Andy the W19/Graphene has a super low sensitivty, 85db,,so the W22 will have same,,,= rules out any dream ofa  845 amp. 
Thats the trade off, I miss out on the 845's dreamy liquid mesmerizing mids in chamber/light classical (= 845 can not reproduce complex orchestra),,but the SEAS W22 + Seas T25002 offers its own magical sound world. 
and can handle both complex thick classical and obviously light classical/chamber.
+ The SEAS T25 voices the human voice like no other tweet I've heard, as it isa  true rare Midtweet has that NEO magnet system. 
I see your Nextel 5.5, has HEX magnet, good, but not Neo magnet system, 
The Neo magnet allow the amp superior power, as the super powerful Neo magnet draws the currect of electrons/=The Siemens,  at a  much quicker pace = less resistance to the amp = cleaner sound. 

Yes the Scan Rev looks likeav  awesome bass response,,But I am not into heavy bass. 
I need midarnge as all my muisc is classical, no bass guitar, no heavy drums. 
= as per OP, Midrange is the most important fq's in any speaker. 
SEAS wins out big time. 
sure the SEAS are 2x's the price of Sacn.s , but if we are going to buy $$$$$$$$$$ amps, why not speand a  few extra bucks and get the most critical compoenet of our systems. 
I've heard $100K Jadis systems on a  YT vid at a  LV show,, sounded like crap, ,,why?
Speakers were 100% crapo. 
Go figure.  
I hope folks realize, big is not always better. Some folks tend to think wow, look at the huge speakers here,,,they MUST BE GREAT SOUNDING,,,haha, Graets ound can come froma  2 way and bow out speakers with 20 drivers. 
I've heard this way back in 1974, The big Macintosh had 25 drivers in each cabinet,,,The interphase A1's  witha  passive radiator woofer +  woofer, anda  tweet<<<< blew the $$$$$$$$$ Macintosh out the water>>>
Go figure. . 

Have you guys seen a  lab called
~~~~SB Acoustics~~~~ Denmark~~~ land of Class AAA speakers. 
??
I just made this monumental discovery last night, 
UNREAL. 
Thor's vs SB Neo driveers,,who will win??
I need 8 months to complete this projecft and will posta  series of vids with the challenge of 
SEAS VS SB Acoustics. 
IMHO it will be a close call, but my guess is SB will over take SEAS 20 yr old technology. 
SEAS is high tech, where SB is super high tech.

Ck out Madisound's kit for SB 2.5