Three things to check. First verify your tuning is accurate by using your tone generator again and watch to see at what frequency the sub moves the absolute least amount. That will be your tuning frequency and in most cases should be relatively close but just be sure. Have you tried moving the sub around yet? Generally gaps like that in the frequency response are due to acoustic anomalies that cause cancellations and such. The last thing to check is all the settings on your HU and amp to make sure you don't have a low pass filter that's turned down in that neighborhood somewhere. Checking and verifying the tuning of the enclosure can help ensure there's nothing goofy with the frequency response from being tuned abnormally low or something. Very uncommon issue, but it could happen. In all likelihood the issue is an acoustic one. You'd be floored by how a few inches or a few degrees of rotation can affect the response of any speaker or sub in a car. They're a horrible environment from an acoustics standpoint. I'm sure you've checked your settings, but hey, shit happens sometimes. There's been plenty of threads over the years where someone has had an issue like this, fought it for a week or two and then discovered something they thought they had previously checked but missed. Verify all that stuff and lets see what you have after that. I'm sure others will have other ideas as well, those are just the first few places I would check. Another possibility is that the sub simply doesn't respond well into that range, but I would tend to doubt that train of thought for several reasons.