Is 48hz really low bass?

I have never seen or used a computer speaker that really truly extended down to 48 Hz - it's a bogus specification to sucker you in to buying it. Most popular affordable bookshelf speakers like the Dayton Audio B652, the models from Mica and Polk fade away pretty quickly below 80 Hz.

The lowest musical note is 32 Hz, which is the lowest pedal on the vast majority of pipe organs (some go down to 16 Hz, but they are rare). A clean 32 Hz is so deep in the bass that you feel it as pressure to your ears as much as actually hear it. The "boom boom" of rap/hip hop is not really deep bass, but more of a higher bass thump.

If you actually DID feel a 48 Hz signal to those speakers, you'd hear them do something... rattle, air whistling, cones making odd noises and horrible harmonic distortion.

No, it's sort-of low bass.

Really low bass is about an octave lower.

Whatever Logitech may claim, you will never hear a loud 48 Hz tone or note from that sub.

"Is 48hz really low bass?"

No.

My main speakers can reach 32Hz and my subwoofer is rated at 20Hz.

What really matters is the roll-off. Although Logitech may claim the Z313's can reach 48Hz, they will begin to roll-off at far higher frequencies, which means anything below around 100hz will be almost inaudible. The claim of 48Hz is what is known as 'marketing'.

The usual range of human hearing us 20Hz to 20kHz. If the note A above middle C is 440, then halve the rrequency each octave you drop - 220, 110, 55. You are now at 3 octaves below A440. Since various factors might interfere with you ability to discriminate pitch much lower than that, these are OK. Subwoofers that reproduce pitches lower than that will make your chest throb, but you will not hear pitch unless you are not human.

Not really, <20 hz is typical low end for good speakers.