"But can anyone explain why a headphone amp that costs a fraction of the cost of one's much more costly main rig would sound better?"
There are no crossovers in headphones. You can obtain high quality SET Class A power headphone amps relatively inexpensively. There are no room interactions for headphones, thus no expensive room treatments. There are no requirements for speaker cables. No requirements for preamp.
There are no crossovers in headphones. You can obtain high quality SET Class A power headphone amps relatively inexpensively. There are no room interactions for headphones, thus no expensive room treatments. There are no requirements for speaker cables. No requirements for preamp.