This is something I have been at for the last few years, I have tried a few drivers, at the moment some of my favorite are the vintage RCA MI 1443, yes filed coil and some funny cone diaphragm, they are used from 500 hz to 5 khz with a Fostex T500 tweeter on top, they do have the non crude or agressive sound, but most of all they sound very organic, they are able to resolve highly complicated music, but they do have that non agressive sound... when called for they cannot go and tear your ears apart or blow them away! And there are times in the music this needs to be done.
My reference at the moment are the Beyma 755. I use them cut from 650 hz up to around 9khz, they can go really high but if you allow this then they start to get fuzzy, I low pass them with a very small coil just enough to get the fuzziness out and let them do their best. They are titanium diaphragms but with a plastic suspension (mylar). I have used and tried for some time a very few drivers that gave me what I wanted, 18sound 2060A with aluminum diphragm and plastic suspension, 18sound 1480 with titanium phrams and plastic suspension. Actually the 1480 and the Beyma 755 sound pretty alike. Now the 18sound 2060 and the 1480 are almost the same driver, 1.4 inch throat and 3 inch phram. The main two differences are the phram material which in the 2060 (or 1460 with extension) is aluminum, the 1480 is titanium and Magnetic flux which is 1.9 tesla for the 2060 and 2.2 tesla for the 1480.
So Mainly two very much the same drivers but one has higher magnetic flux and titanium vs aluminum diaphragms both with plastic suspension.
Soundwise both are very good drivers, but the 1480 is much better. the most dramatic change between them is the weight of the instruments, the 1480 has much more impact and the instruments seem to have more weight to them, they are more easily identifiable with better definition and separation between instruments. This means that with complicated music, I can think right now of the Russian Easter Overture the 2060 plays it with no flaws, but the 1480 puts more shine on the trumpets and better weight on the instruments meaning also more separation and transparency, not very noticeable with simpler music, but when the crescendo gets here, you will want the 1480. Now the beyma 755 has all the same attributes than the 1480, titanium diaphragm plastic suspension with 2.2 tesla, no wonder they sound very similar, as resolving and with the same weight on the instruments I have come to love, but the tone of the Beyma is a little bit more organic, instruments are little more credible, you believe they are instruments a bit more than with the 1480, and that is important.
18sound has since come out with a new diaphragm in the NSD1480 that is titanium nitride coated, whatever that means it is supposed to have 6 times more stifness than normal titanium and twice as much a berylium, I call it the Viagra driver, but I havent been able to get a pair for testing yet... and my urge to test drivers is decreasing.
Of course I started out with a pair of Altec 802 which were surpassed easily by Beyma 380 with plastic phram, I latter hated both. I tried JBL 375 bubblebacks and did not really care for them, they were nice on piano, but wouldnt go high enough to make a credible violin and couldnt solve a good as the 2060. I got Radian 950 which I thought at the moment would be a more modern version of the 375 with plastic suspension on the diaphragm and neo magnets, but I found that one shouty, I went to a lot of trouble to tame it, and in the process getting some of it good attributes out of the way. Bruce Edgar says there are drivers that work better on different horns, I guess they didnt like round tactrix MDF horns.
I dont know if the case is the same for the TAD 2001 drivers. on my tactrix horns they just didnt sound right. I tried them in 2 different horns made for them, and the results were always the same. They have very good soundstage, I guess it was becasue I let them go almost up to 20khz, or whatever they could go up to, I didnt low pass them at the begining. For simpler music you will have trouble finding a nicer driver, female voices were very organic, foscused and with great soundstage (because the tweeter was disconnected) They dont go all the way up, but high enough that you wont miss highs. Of course the UHF is great to integrate music, I later added the Fostex t500 way up high. That being said there was always a whizzer sound present. I know most people are not sensitive to say Lowther whizzers, and they have a big popularity, I lived and loved lowthers some years back, until one day something clicked in my head and I started listening to the whizzer sound, I didnt have the guts to cut it off, so I went back to my 802, which killed lowthers down to 800hz. I even asked a few friends if they could identify the whizzer sound and couldnt, I envy them. Now TAD 2001 exhibited the same sort of whizzer sound, but at a higher frequency, not very noticeable in the begining but with time it came to a point when I just could not bear it anymore. Of course I could fight back with lower xover points and certain tricks, but the truth is , they are not very good drivers to beguin with, not resolving enough; I would put them next to the 2060, nice and sweet but when the crescendo comes on the Polovtsian Dances and the chorus goes in full force, they shy out, just cannot make what the Beyma 755 does with flying colors, even distort on climax with 1.5 watts.
I turn my own horns out of sadwiched MDF and have made at least 10 different MF horns by now, I try to match the driver with the exact horn size according to measurements and listening tests, IME horn loading makes a bigger impact than Xovers, if you have a bigger horn and try to cut your drivers higher, they wont listen to the xover, they horn will control the cut off frequency most likely.
Right now I am playing with JBL2490 drivers and when a good low pass is added they can go real low, I have them on 120 hz horns, have not measured them again lately but I can tell they are loosening up nicely. I want to get some diaphragms for them with plastic suspension, maybe that will help them go just a bit lower to blend in perfectly with my lower channel.
I am still looking to try the S2 drivers, which I heard on Romys old system and loved them, of course, and GOTO 370, just to see what the fuzz is all about...
At the moment I guess I can say plastic suspension is a very good thing, and high tesla, 2.2.