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

As for the mention of the Thor, I do have a bit of a soft spot for the Seas Magnesium drivers. They have a unique warmness on the mid and lower mid but very natural, and at the same time, very detail like a hard cone. It took a bit of time to break in and probably may be the longest of any drivers I’ve had. At first they sound a bit cold and clinical, but once fully broken in, it’s all fine. They have a bit of a break up so just be mindful of the xover.

I currently have a pair of speakers that use the Seas Nextel 5.5in driver and the magnesium has the exact the same dimension so it’s a one to one swap. Hm... I have might an itch to get a pair.

But is the mid more important?  Probably but it needs the right tweeter to make a perfect dance partner.  

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. 

I have nothing against the Magnesium Seas line, but I have to be a fanboy and plug the Scanspeak Revelators which is what I use. Warm, and in the right (i.e. large!) cabinet, have amazing bass for the size. If you are doing a 2-way I highly recommend them. :)
Of course, this is a hobby and fun so pick what you like, I just reserve the right to be a fanboy from time to time. :)

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. .