Me x3
Member of the Trade: FiiO Store Argentina
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If you do the math, the Amp would need a very high output impedance to create this effect. An Amp with a 10 Ohm output impedance would only reduce the level at a 300 Ohm load by 0.282 dBV, If the impedance goes up to 600 Ohms then it would reduce the level by 0.140 dBV. That's a difference of 0.142 dBV which is inaudible. Change the Amp's output impedance from 10 Ohms to 100 Ohms and the difference from the same two load points becomes 1.16 dBV, again not very significant. IMO don't buy an Amp with an output impedance much greater than 100 Ohms. Most Amps have a far lower output impedance, so I wouldn't lose sleep about my HD600.
I think 100 Ohm might yield audible results, it's a wide region between 45Hz and 250Hz where you get a slight boost (0.5dB < boost < = 1dB). Very subtle, that's granted.
That said, you can find higher than that. I had a modern Yamaha A-S500 integrated stereo amplifier and the headphone output was 470 Ohm. Great speaker amp for the money, btw.
On top of that, high levels of second order distortion is the bass increase the sense of bass quantity.
(40Hz + nothing) sounds different than (40Hz + audible 80Hz)