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

the hardest frequencies to get correct are 40hz-250hz in the mid and upper bass.

(1) it’s the power frequencies where vocals, drum kits, cello’s, and piano’s either live or die. (2) it’s where the room will have the greatest things to say and (3) where your amplifier will be the most stressed. (4) it’s where you likely have a crossover. (5) it’s where you need linearity, but (6) are typically short of headroom in driver surface. so (7) excursions are excessive.

micro-dynamics, slam, and explosiveness in the music is in these frequencies. the 'life' of the music. 

and the higher the dynamics of the music the greater the failure in this area. big music requires this part to be fully sorted out.

which driver covers this range varies from speaker design to speaker design. mostly it’s 2 drivers, or multiple drivers of two types.

in the most ’uber’ well thought out systems it’s this part that gets fully done. lots of systems get the mids right. this is the hard part. and the break thru area of ultimate performance.

and this is where alternate driver types (horns, panels, omni) fail. they struggle with coherence here.

Eric Alexander of Tekton uses multiples of tweeters (14) from 270 Hz up to 2500 where he crosses over to a designated HF tweeter which is the same as the 14 used as midrange drivers. So in his flagships with 15 tweeters the same driver(s) are both midrange and tweeter.

@ 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

 

I find that the most discerning/challenging speakers have a low impedance point in the 100-150 Hz range.

The Trym's impedance in this range is nice at ~ 8 Ohms or higher throughout the bass. Should be an easy load, with plenty of strength there.

Based on the frequency plot, it will benefit from being 2' or closer to a rear wall.