Well, I don't know when the hum becomes audible relatively speaking, but it strikes me that it might also be nothing more than the noise floor of the phono stage interacting with a cartridge with low enuf gain to cause you to crank it up to get sufficient volume. And, perhaps the more obvious, you are initially hearing the onset of feedback caused by excessive volume as a low level hum. Deep bass creates a lot of air movement, some of it subsonic, and that could be causing the TT/Arm/cartridge to resonate. Check this out by listening for hum increase when the stylus is not in the groove and see if the hum increases with volume.
Hope that helps a bit. Solving problems long distance is a bitch. :-)
Hope that helps a bit. Solving problems long distance is a bitch. :-)