It could also be that the sub is making audible what was already there but unheard (because the owners speakers don't reproduce the very low frequencies that the sub does)---the low-frequency resonance created by a mis-match between cartridge compliance and tonearm mass. A cartridge and arm need to be paired such that their combined low-frequency resonance is located between 8-12Hz, 10Hz being optimum. If the cartridges compliance is too high for the effective moving mass of the arm, or conversely the arms emm too low for the cartridges compliance, the resulting resonant frequency of the pairing will be too low, below 8Hz. The lower the frequency, the worse. A very low-frequency resonance can cause tonearm instability, power amp distortion, and woofer pumping.