Now that I'm back at the house... that suckout between 35 and 50 Hz is big enough to drive a truck through. At 20dB deep, I would expect it's audible.
It screems Acoustics to me... likely a null in your room at the LP... could be a cancellation due to soundwaves bouncing around... perhaps a combination of things...
No two ways about it, though... I would look for a new home for that Sub!
Having a second, of course, will help smooth that stuff out, too, but if you are trying to get the best performance from one sub, the Crawl will help you find a better spot!
Do the Subwoofer Crawl. It's the easiest way to find the proper placement location for your subwoofer to get the best performance and sound.
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