In regards to the original post, the TD15H "can" play to 1600hz. The response of the TD15H and TD15X are mostly the same other than the low end since they use all the same parts, only a different VC. This changes the Q and affects the low end response but has little change in the upper end. You can see augerpro's measurements of the TD15X here:
Frequency Response - drivervault
Impedance is also very flat and extends to nearly 10KHz before the resistance doubles from it's nominal value. The unchanging Le with respect to excursion also is a huge benefit. Without this constant Le, the upper end of the response curve varies with excursion throwing off any crossover, specifically in a passive system, but also in an active.
Impedance - drivervault
That all said, look at the off axis response by 1600hz. As with any 15" driver the response will be varying as you get farther off axis by that frequency. This really determines the upper end limit more than anything else. At 20 degrees off axis the response at 1600hz is only down about 3dB but by 40dB it is around 10dB down. At 1KHz though it is much less. Only about 1dB down by 20 degrees off axis and only about 3dB down by 40degrees. This is much more manageable and a more realistic upper end limit.
For a 2way, there are several options to go with in waveguides/horns that will allow you to mate to the TD15. My favorite option to date is not actually a 2way as it uses a coaxial compression driver. However that portion can be done passively. The 18sound XT1464 with the BMS 4594ND is very difficult to beat. This compression driver fits on the horn and has a 1.4" throat and the large diaphragm covers up to 7KHz range where the smaller diaphragm takes over from there. I have yet to find a compression driver that can offer better performance.
Overview
The only change I would make would be to go with the TD15M instead of the TD15H if you do not already have the H. The extra efficiency and extended response are beneficial in a system of this type.
Regarding Neo, the only benefit with going to Neo over ceramic is weight. In a pro system with many drivers, this can be more critical. For a home system where it won't be moved on a regular basis it has no real benefit. With a neo motor we could get a TD15 from about 27lbs down to around 18-20lbs. A radial neo has the benefit of much lower flux modulation as the strong force of the magnet is right next to the VC. The same went for the old Alnico magnets that were placed in the pole right next to the VC. The full copper sleeve on the pole is just as effective at reducing flux modulation but has the additional benefit of linearizing inductance with respect to excursion.
The only further way to get more linearity is to linearize BL, which is what an underhung motor will do. Our Dipole series all take advantage of the underhung as we want a higher Q but more linearity as the dipole drivers require more excursion to compensate for the baffle losses.
John
I am meeting too many people who believe that the best days of hi-fi are behind us. 
