Low-frequency distortion refers to the alteration or distortion of audio signals in the lower frequency range. In the context of amplifier design, low-frequency distortion can result from pushing the bass end of the amplifier's frequency response to its limits. When the bass frequencies are amplified to a point where they exceed the amplifier's capabilities, the power stage may struggle to accurately reproduce the waveform. 

Cranking up a vintage amp like a Fender Tweed or a Marshall JTM45 with the bass turned up all the way will make the low frequencies push the power stage into significant distortion, possibly hindering its ability to fully recover between notes. 

This makes the sound thicker and more full-blooded - which vintage freaks will appreciate. At the same time, it will lead to a muddy sound, lacking fidelity and sometimes even losing harmonic content. In the aesthetics of many modern guitar sounds, however, this phenomenon is rather undesirable.  

That's why more modern guitar amp designs have moved toward generating tighter, more precise overdrive by limiting low-frequency distortion. The output stage remains tight and responds much faster than the power stage of a vintage amp, allowing even short and quickly played single note riffs - possibly with drop tuning - to be reproduced quickly and accurately.

By the way, the two BluGuitar AMP1s serve both sound aesthetics: While the AMP1 Mercury Edition is tuned more towards vintage sounds (loose and fluttering power amp with a certain degree of low-frequency distortion), the AMP1 Iridium Edition's sonic focus is on tight metal sounds (fast and precise power amp with limited low-frequency distortion).