There's some truth to the idea - below a certain frequency, we can't localize the sound. This is just true, neuro-physiological fact. It can be shown in controlled conditions, and the psychophysics of why is well understood. It's a bit squishy where the transition is, but the common crossover setting around 80Hz is in the ballpark.
This is why a well integrated sub doesn't need to be centered exactly on your TV, or co-placed under/on your front speakers in a dual setup.
This is NOT the same thing as "a sub can be placed anywhere, it doesn't matter". Subs can add high frequency distortion which is localizable, a sub can interact with the room in ways that add "boominess", the sub can transmit vibrations to the cabinet it's up against, and all sorts of things happen in real-life that won't happen listening to carefully controlled sine wave sounds in an acoustically controlled room.
Yes. A poor quality subwoofer will in normal conditions produce harmonic distortion that are multiples of the input signals, and those multiples are often high enough frequencies that the subwoofer can be located by sound alone. The crossover setting will not change this fact. And even a good subwoofer, if overdriven, will produce such distortion, so pretty much any subwoofer can be located by the sound alone under the right conditions.
To properly test the matter, one would need to be led into the room "blind" without knowing where the subwoofer is. Probably the easiest way (though the most expensive) would involve purchasing a dozen or so subwoofers and putting them in various places in the room, but only actually using one of them at a time, with someone hooking up a different one when one is out of the room, but with wires running to all of them. But even then, one must not be able to see the actual woofers, or one can tell from sight which cone is moving on loud passages.
If one is really locating where the subwoofer is purely by sound alone, then either one has improperly placed the subwoofer (e.g., put in on something that is not solid, like a TV cabinet that vibrates, etc.), improperly set it up (e.g., set the crossover too high), it is being over-driven (so one needs a better subwoofer that can play as loud as one requires without such distortion), or it is a cheap piece of junk subwoofer. Since many people set their subwoofer level unnaturally high (to get an extra bass boost) and listen at very high volumes, they need an incredible subwoofer to have something that they cannot locate by sound alone.
Also, the THX standard of 80 Hz was selected with the idea that one would be listening to movie soundtracks, not to test tones that are limited to subwoofer frequencies. So the question is, can one locate the subwoofer by sound with actual music and movie sound, or is it only when one is playing a test tone for the subwoofer? If only the latter, then there is no real problem.
