I'll be using the Hypex Fusion plate amps in my design so I'm not limited by the natural Fs in my design as the Fusion amps have built in DSP I'll be using a Linkwitz transform to lower the -3dB point. My design is primarily for a nearfield / midfield monitor, it will provide enough SPL for me even after the Linkwitz transform.
With this in mind I think the new XM001-04 L26ROY is just about the perfect woofer for closed appication. What is important in the woofer is the SD and the xmax ofcourse but also especially how linear the motor force is under excursion (so how clean it stays under excursion). The new L26ROY excells in this. It will really give clean bass up till right about its limit also under Linkwitz transform in a closed box.
I have the woofers here already and tested them and they are amazing, truly clean and deep and in control. Had looked into other woofers before that but they dissapointed.
Btw here are Klippel test results of the new L26ROY:
https://audioxpress.com/article/test-bench-the-l26roy-10-subwoofer-from-seas They are truly good.
Also the L26ROY is not just a sub, it can be used as a woofer as it stays clean enough and doesn't have fr problems >100Hz or too high a inductance (which modulates with excursion) like many subwoofers. I plan on crossing mine at about 180Hz still soubting between LR4 or LR6 (will determine final crossover freq and steepness from testing).
If the driver you're looking for is for sub duties only then many more drivers become available, but for my application I don't think there are many other options which are as good as the new L26ROY.
About the T34A-4, the way he got the off-axis so amazingly good is by the geometry of the tweeter, it is extremely curved / round. Though I think the real magic was how can such a large tweeter behave so well up to 20kHz, this was achieved by making it very light and unequal thickness across the membrane this is really a tweeter from the future
It is a bit expensive (I paid about €265 per tweeter inc shipping) but indeed as you say not crazy for the things it does (I think it's even cheap for that actually haha).
Btw, don't mind Dickason's measurements of the T34A-4 he did them badly I think under not good circumstances and his measurements show that, Troels and Hificompass made much more clean measurements.
I also have already tested the tweeters myself and they sound jaw dropping amazing
The transparancy and dynamics are the best I've personally heard and that was only on a test baffle yet which still had diffraction.
One other thing which is not often looked at in speaker design is that our ears are so much more sensitive between 2-5kHz and I've found that any crossover effects in this range sound the worst, they make for harsh sounds and destroy transparancy for me and just totally mess up the whole smoothness and balance of the treble to me. With the T34A-4 one can cross low enough that all crossover effect are not just very far off-axis but also stay below 2kHz. Our ears have a sensitivity dip between 1-2kHz and I've tested the effects of this and the crossover dip is just so much less noticeable between 1-2kHz and doesn't make the treble sound bad (I find it mostly makes things sound a bit further away which isn't that bad a thing). In my personal view of things the treble "sits" on the very important 2-3kHz range, I personally call that the lower trebble. I make sure that part is completely flat and smooth also off-axis. Completely unaffected by the crossover or mid-driver dropoff or by diffraction. I have heard the effect of this only in simulations on the computer (where I run audio through it and listen with headphones to the sim), to hear it for real I have to finish my speakers
Edit: oh btw, about intermodulation distortion. Since there are almost no measurements of this one must go by the standard HD measurements. There are two general rules here. If the driver has a rise in its fr this will also amplify IMD products, and one can deduce IMD amount by the relation between the 2nd harmonic and 3rd and 5th harmonics. It is especially important that the 3rd and 5th harmonics are very low. not so much for the 2nd harmonic. So the T34A-4 may not look to be too impressive with its 2nd harmonic level, but that is inaudible in reality what's important is that its 3rd and 5th harmonics are the lowest there are, and that it's cone breakup / rise is above the audible range.
And for the mid driver this is where it most often goes wrong. Most mid drivers will have levels of 3rd and 5th harmonic which are not particularly low and rising a bit till the crossover point and then they often have a beakup mode or rise close to 5kHz. Even though you cross the driver below 5kHz this does not matter at all for the HD and IMD products falling around 5kHz they get amplified all the same. The SB15CAC30-4 is simply amazing in this, it has extremely low 3rd and 5th harmonic, and a small drop in sensitivity around 2-5kHz no rise at all. The result is the cleanest mid driver I've ever heard, also extremely linear. On a wide baffle where the baffle step occurs below where you cross the mid driver it can go plenty loud too / plenty of dynamic range. I think it's a steal for the price when used this way
I'm so happy with it. (for a higher more normal crossover point it's perhaps not the best, it has a bit of a dip at around 2.6kHz in the off-axis for instance, etc. but that's all not a problem when crossing it at 1.3kHz with the T34A-4)
Btw, the SB15CAC30-4 has it's main cone breakup / rise in the 9-10kHz range. This is where we find another dip in the ears sensitivity / equal loudness curve, and it's far enough away from the crossover point so that hardly any distortion product fall on that anyway so this is really the most perfect metal mid-driver there is in my opinion
It's like it's psychoacoustically perfectly made for the human ear.