20hz is hard to hear. Your 20db more sensitive to 40hz and 35db at 80hz the first 2 distortion products. Also speakers have the most distortion at 20hz, those speakers are down 50db at 20hz. And some tone generators have a lot of distortion. Found this about the subs "The SB-1000 Pro can be pushed into a hefty level of distortion atlower frequencies, and that is expected since the entire onus of airdisplacement rests on a 12” driver with 13mm of Xmax, and that will reach itslimits relatively early in deep bass below 30Hz."
So yea i think your hearing distortion, but this is normal at 20hz at high SPL which is the only way you'll hear/feel any 20hz. There's almost no 20hz in music and with movie sound fx, if there is any 20hz, the distortion wont matter. Do you think your lacking bass? I don't think you have a problem.