Originally Posted by elmoe
Most of the time the only reason to worry about output impedance is going to be when dealing with OTL (output transformer-less) tube headphone amps. These amps can sometimes have a high output impedance which will result in a lower frequency drop with low impedance headphones. Unless you're dealing with an OTL tube amp, you don't really have to worry about it.
If you want to be sure, apply the 1/8th rule. The amp's output impedance should be at most 1/8th the impedance of your headphones (so if you're for example using Grados which are 32ohm, you want an output impedance lower than 4ohm).
Sorry, but this is not technically correct. The reason an OTL amp would drop low frequencies is because all OTL amps require output coupling capacitors to block the DC to the load (headphones). Unfortunately, the coupling capacitors and headphone load resistance form what's known in electronics theory as an RC circuit. Such a circuit is frequency dependent on passing current. Long story short, if the coupling capacitors are not big enough with low-impedance phones, you will get a loss of bass frequencies. Typically, coupling capacitors get enormously expensive at high-voltages (typical with a tube amp), so it's not unusual to see capacitors that are sufficiently sized for bass frequencies of a 300 ohm headphone, but not so well sized for headphones that are lower in impedance. However, the fact that the design is OTL is not the true cause - only the selection/expense of the coupling capacitors.
Similarly - and something no one's mentioned - solid state amplifiers most often run on very low voltages. A high-impedance headphone is going to want to see high-voltage differences to perform well. This is typically on the order of +or- 12V to 15V or more. Very few typical solid-state amplifiers can supply that unless they are a high-quality desktop SS amp.
A true, output-transformer-coupled amplifier is optimized for both situations of low and high-impedance. Depending on the design, this might be a Hi/Lo Z switch or, it could simply be two different headphone jacks wired to the different windings on the output transformers. It's absolutely the same thing - switch vs. secondary headphone jack. It's just a design decision in the end.
Anyway, hopefully that cleared up some things.
Edited by tomb - 6/14/14 at 4:16pm