fewtch,
Sure, I'll give it a shot. It's always fun for me to be corrected.
An IC amp usually has about 24V voltage swing (+,- 12V), a transistor amp has about 200V voltage swing (+,- 100V for a typical 100 watt amp), and a tube amp about 600V voltage swing (+,- 300V).
When you put a 30 ohm or 300 ohm load across an amp output, you typically have a voltage drop across that load; in this case the driver.
The problem comes in when one tries to minutely adjust the volume. On an amp which is made to drive high impedance loads, since there is such a high low-noise to volume floor, it can be more precisely attenuated. The less efficient the better as it will allow even more minute resolution. Any transients will have more dynamic headroom.
When you play a low impedance headphone through an IC amp, since you only have a certain amount of voltage swing (if you are using a 9v battery, you basically have +,- 4.5V in class A or the signal is moved up to the 4.5V (0) reference line, thereby giving you a 9V voltage swing), the headphone must therefore be much more sensitive to fluctuations in voltage changes, and must therefore, by necessity produce an appropriate change in signal strength. But your noise floor is very low (high-noise), so it could be noiser and also sound more compressed, especially if a transient needs all the voltage. (if ground is your noise floor, you basically have +,- 12 V to maximum voltage. So a change in voltage will produce a corresponding change in noise floor). You would therefore find the highest note on a CD and then adjust the volume so that that transient doesn't sound rounded or clipped or compressed, and then start the real listening. As today's CDs are becoming more and more compressed, what you end up with is a muddy mudge of smudged mud. You will lose a lot of the dynamics, but the overall balance will still be there - loudness.
When you play a high impedance headphone across an IC, there is a massive voltage drop. (look at any amp spec, and you'll see the wattage available drop as the impedance goes up). It's the available wattage which will dictate how loud it can be played. Since a high impedance headphone has a lot of dynamic headroom it will tend to sound thin on IC amps (not much of the height (dynamic) available will be used because the amp is narrowly supplying the voltage swing. The "woofer" needs more voltage to get the cone really pumping air. In this case the "bass/woofer" may be over damped, or limited. The cone can only travel so far, but because most of the energy is being used to drive the "treble/tweeter) it will tend to sound bright. Basically the balance of the sound is off. Here a change in voltage will not produce a corresponding change in volume level perceived. It'll be like it can never get loud enough. This perception comes about because the tonal balance is off, due to the change in reactance (DCR). There is basically a phase shift within the music.
When you use a low impedance headphone on a high impedance amp, the headphone is still too sensitive - it will more readily go into distortion, and chances are the bass will sound flabby, as the current necessary to control the voice coil surround will not be there (unlikely on better amps, as they tend to have better damping factors). You may also notice that it reaches a certain volume level and then quickly goes into distortion past that repeatable volume level. Now the opposite of a high impedance / low voltage swing is possible - the headphone could sound too bassy.
Now since ICs are low voltage devices (typically 5V), you would need a transformer to jack up the voltage. but a transformer takes up more room than the IC, so it is impractical for all intents and purposes. With high voltage / low current transformers, you need more thinner wire (higher guage), with low voltage / high current transformers you need more thicker wire (low guage). You are now in the magnetic density / square micro-inch territory. You need a certain amount of magnetic density to move a voice coil. So for the same amount of space, you can jam more thin wires into a high impedance voice coil bobbin. This will give you much more uniform magnetic density, as the wires will occupy more empty space around the magnet assembly, since the wires are rounded. With thinner wires you would have more resistance, could carry less current, but what about inductance and capacitance? same for low impedance voice coils, what about inductance and capacitance? And for both, reactance?
just as you could bring out the high end on a dark headphone by putting a resistance in parrallel (in which case current is being sinked into ground), you could also tame the highs by adding a resistance in series with the hot lead. in both cases the load on the amp is seen differently. would I want to put a 10 ohm resistor across a HD600 to make it play on a IC amp? not really, because the voltage swing may not produce fine transients. it will probably sound much brighter. why? see above. you may perceive that the bass went away, or conversely that the treble was boosted.
So, it's all in the design and engineering. An amp can only produce so much power (wattage). The voice coil in the headphone is engineered to have certain characteristics within a certain power band. As far as efficency goes, there are at least two different standards which are used to measure. They are usually measured at 1KHz, so it has no bearing on bass. So a less efficient headphone which plays more bass will sound louder than one that doesn't play as much bass but which has a higher efficency rating.
Now, about music... rock vs classical... just listen.
btw,
ReDVsion, I run a DT831 through my soundcard, and on my other computer I run the SR80. best thing to do with either is to take the soundcard out to a receiver. then you have the receiver headphone output to worry about. but the hiss is gone and the bass suddeny appears. a soundcard probably does not supply ample current nor voltage to make a headphone sound really good. but it is acceptable - you just won't know what you are missing.
if you look at the specs for the Aiwa AK100 you'll see that it has low impedance (16 ohms) and high effeciency. too bad it has no bass. it looks a lot like the Sony F1.
So it's either go with a portable type of headphone (V6, D66, PortaPro, KSC35,etc.) or take a chance on Ultrasones, K240S, A900X or DT531. But for $200 I would definitely try to find a place where I would return them if I didn't like them. Once you move them off your soundcard the sound will completely change.
