Okay
Most of these compression drivers when used in CD horns like the 18 Sound XT1464 either a passive eq or active eq to flatten the response.
You may have mentioned earlier but do you have any form of RTA or measurement kit?
There are some reasonable diy options at Parts Express.
Attempting to optimise CD horn passively is difficult without an crossover software package with an optimiser and take some skill.
For diy home audio all CD horns require an optimised eq contour as they tend to have constant directivity that means in the non eq state the on axis response looks like a banana.
The iwata is more diy friendly but will still require some intervention.
Many years ago I had a JBL 2397 horn and I took it to a friend who was running a college loudspeaker builder course and he measured the horn for me and built the crossover for the 2405.
Depending on where you are that can be a blessing if you can reach out and find some assistance.
If that is not an option you can self help and look at what the 18 Sound people do with this particular horn to make it function correctly.
A starting point is the 18 sound web site enclosure design tab. If you have a look at the 15 inch 2 way it's quite intuitive process. If you are keen as a starting point just use the eq part of the 18 Sound crossover and place a 1.5 uf capacitor in parrellel with a 27 ohm resister. This will be approximate but will be in the ball park as to normalising the CD characteristic of the XT1464 horn.
The effect of this is a response shaping network. It woks by attenuating the response from a selected high frequency in the form of a high pass filter . At the same time the lower end of the response has some sensitivity shave off
Most compression driver work on the same principle so you are not doing anything wrong by trying this.
Still use the large series capacitor before the above eq and crossover with the Marchand crossover at 1500 Hz. I would also suggest a fuse in series to protect the driver in the event of an accident.
The Lpad will not be required.
Measure the response if you can or listen to the horn with Pink noise (or an off station FM radio) and compare to the non EQ version of the crossover. Do not connect the 2405 at this point.
The horn should sound smoother and more extended.
You can then add the 2405 back and compare.
The horn will extend further but will tend to sound soft in the upper hf area. This is normal and I would listen carefully on some know program content .
Adding the 2405 will add spatial resolution with a crisp crystal quality.
I see
http://www.eighteensound.com/Portals...15%202ways.pdf
Let me know how you go