how is a headphone amp better than plugging them into say, a 100Wx5 receiver?
There are pluses and minuses to everything in this world....
Since you've asked....
Headphones would be free of unwanted room acoustic effects, reflections, and cross-talk.
Some feel that we only have two ears, and multi-channel simply adds complexity in attempt to cover-up bad recording technique.
Doubt me ?
Playback with headphones.
Play a recording made near-coincident with which you slowly opened the spread-angle of your mics.
The image will travel from front to behind you.
Now narrow the angle.
The image compresses into the center of your skull until you are listening to monophonic sound.
Four speakers are not always necessary.
The fifth is simply compensation for poor bass performance in speaker design.
Subs will corrupt transient sound placement in the LF region, but most consider the effect of little consequence.
Bigger is not always better, less is sometimes more
YMMV