Hi ChuckAuger, it has come to my attention that you have some questions/concerns about the TD18H's that you received. This is the first I have been aware of this situation. I have seen no mention anywhere via email or post on my forum that you were having any issues at all. If you contact me via either of those methods, I will get back to you. It may just take a little time.
Most of the popular pro18's have similar response curves over their main target area, below 100hz. They are all optimized for similar enclosure size and low end extension in a vented enclosure. Vented enclosures from 6-8cf per driver tuned from 30-40hz are most typical. The low end of the curve is primarily controlled by the enclosure itself. Mass and motor strength along with the Sd are the main determining factors from the driver side. The TD18H was designed to meet these same target points as the other drivers in the industry and be able to exchange easily into many of the common enclosures out there. The following model shows a dual18 cabinet, 15cf enclosure tuned to 30hz. The TD18H in in green, B&C18TBX100 in blue and the JBL2242 in red. I could have easily put in the EVX180B or others into the comparison as well. As you can see, below 100hz the curves are all quite close.
All drivers with 2000W input to the pair have reached about 10mm Xmax. The JBL has the most Xmax due to the lowest DCR of the drivers. At 4.7ohm DCR, this is quite low to be considered an 8ohm nominal driver. One thing to note is that the B&C and JBL have surpassed the 9mm rated Xmax at that input power while the TD18H has substantial headroom left before the 14mm Xmax. The suspension of the TD18H actually allows for almost 25mm one way travel before damage as well. I have not done a side by side measurement between the JBL and the TD18H myself. I have done so with the B&C18BX100, EVX180B, RCF18's, etc and they have all tracked very good with the models. I will be honest that the intent of these measurements was all focused below 100hz.
I'm not sure what your alignment is, what the measurement methods are, or what frequency range you are seeing this difference in efficiency of 5dB. If it is up above 100hz, the inductive bump of the 2242 (2.7db at 200hz) could be a partial factor. This is predicted in the model above though so it should not be much of a surprise. The JBL clearly models to be more efficient in this region. However, the TD18H models to be flatter and have a further extended high frequency response. The priority though for live sound subwoofer use was on the low end. The 9mm vs 14mm Xmax allows the TD18H a full 4dB more output. For this reason I have many TD18H's out in the live sound world now that have replaced other 18" drivers that had been prone to damage. These installers and users have also confirmed the same things I have in measurements here.
I would be very interested to see your measurements, methods, and the details on your enclosures as well. Without knowing these details it is difficult to understand what it is that you may be seeing.