Alright...Let's see if this helps.
Below are some basic ideas or variables to keep in mind when looking at distortion levels during the output compression sweeps and attempting to compare different units to each other.
Above is the basic response of 3 quite different subwoofers measured with the same equipment in the same manner. The vertical direction indicates increasing SPL level or loudness. The horizontal direction indicates what frequency in the bandwidth. The left side of the graph is deeper frequencies and the top of the chart is higher SPL level. (Just in case anyone reading is not sure what they are looking at on these charts.
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Notice how different the response shapes are. Also remember that we calibrate the subwoofers to a baseline of 50Hz at 2 meters outdoor groundplane. As can be seen that is where the lines all converge is near 50Hz but as we get further away from that point the responses deviate greatly which results in one subwoofer producing much more output in the deep base and another producing much more in the upper bass range. Looking at these 3 subwoofers it is apparent that the green one is being driven the least hard of all at the baseline sweep level since it peaks near 50Hz. The black one is being driven the hardest overall and much much harder than the other two in the deepest bass. As can be seen at some frequencies the subs can be outputting significantly more than 90dB and at others much much less during a 90dB nominal sweep.
Note that 50Hz is chosen as the reference point to level match them because it is an area that almost every subwoofer is strong at and it is smack in the middle of the typical subwoofer range of operation.
Below we will see what happens to the responses when the output is increased 20dB to a nominally 110dB level.
Above are the results for the same subs driven 20dB harder. Note that the responses have changed quite a bit due to compression and limiting and there is still a lot of deviation. There is also starting to be a lot of drift away from each other at 50Hz where originally they were very close. This type of thing is why one subwoofer may be compressing already on a 105dB sweep while another may not until driven much harder and in some cases different sweep levels as indicated might actually be a better comparison of overall output levels than the same nominal sweep level.
This is also why I usually say at a "nominally" 110dB sweep. This means that it should be producing 110dB at 50Hz not that it will be a flat line over most of the range because many times that is not the case.
Above is the THD or total harmonic distortion from the Green and Red subwoofers during the nominally 110dB sweep. Note that the Red distortion levels are higher in the upper bass but if you look at the 110dB sweep results from the middle picture you should note that the Red subwoofer is actually producing 10-12dB higher output over much of that range. That is the output of 4 units of the Green sub! The Red subwoofer also produces much higher output in the deep bass even though the distortion levels seem close. This type of thing is why you have to look at more than just the THD% for a useful comparison. One subwoofer may be producing the output of 2 or 3 of the other subwoofer over a range in which case a direct distortion comparison is meaningless.
This is just in order to compare our own results against each other. I'm sure you can imagine how difficult it will be to compare results from completely different types of tests to each other in a fair manner.
Regarding the questions of not being able to compare the PB13-Ultra THD to the Velodyne DD18+. The above can show how this is more complicated than it seems. Also The DD18+ I used the 115dB sweeps for it's THD chart where I only showed the 105Db ones for the PB13-Ultra. The reason that I decided on that level for the PB13-Ultra is because it is really 3 separate system tests with the 3 different operational modes and as such the review would have been ridiculously jammed with measurements so we had to condense things down. 105dB was chosen because that was the highest output level before significant compression from the sealed mode and where the output levels match ed the best. I also assumed that most users would want to compare THD when the sub is not being asked to give everything it's got. I wanted to compare the distortion in that manner so that is why 105dB was chosen. You can see the THD data for the higher and lower output levels at Data-Bass where all of the measurements are hosted. Normally I would show THD at a couple of different sweep levels but the PB-13 review was so large already that we had to condense things a little.
