Yeh, I see that alot too. Because of context I've always figured it to mean anything below about 50hz. Which is, by some, supposed not to be reproduced by a guitar amp. But in practice you can actually hear the difference between a .047uf and a .022uf decoupling cap off a standard gain stage. There must be at least some fundamental interaction due to LF response in the amp that affects the final tone even if the your not getting much below 70hz out of the speaker. So the idea that just chopping off the "sub lows" will only eliminate non audible frequencies, and not be noticable otherwise has to be wrong. I have noticed that anything above about a .0047 cap feeding the input of a standard LTP PI seems to make no difference. But that's because of the impedance relationships in that circuit.