at first, words of classic
If you want to know a headphone's efficiency, look at the sensitivity rating, measured in Decibels (dB). You'll find how loud it gets with the application of a certain amount of power (Watts).
Impedance tells you how efficiently power transfers from the amp to the headphones.
Amplifiers have an output impedance. The difference between the output impedance and the headphone impedance tells you how well power transfers from the amp to the headphones.
When you know how well the power transfers, then you can use the amount of power that gets through along with the sensitivity rating of the headphones to calculate how loud the headphones will get.
There's more to this (of course), but that's how it works. Generally, you want an amp with an output impedance lower than the impedance of the headphones. The lower the ratio, the better the damping factor. A high damping factor gives you better control over the drivers.
You don't want to do something like drive a 62 Ohm pair of AKGs with an OTL tube amp with an output impedance of around 200 Ohms. That makes a mess of the bass and sound quality.
and no me, young gun
Volume level, depends on source, in my case. From DIY amp, around 25/100, from MP3player (cheap one) 25/32. And actually, times before, when I´ve listenig with KOSS KSC55 (60 Ohms!!!), it was still about blasting my volume almost at max, but now - despite the fact that HD25´s phones have even higher impedance (70 Ohms)- it´s an opposite, decreasing of volume. So, I could said, It´s not about an impedance, its about SQ which is provided by headphones.
And YEAH, sound-(BASS in this case)-it´s individual (like everything) and depends on preferences, but there´enough BASSes on HD25´s, they don´t lack bass, no way. But on the other hand, they aren´t BASS HEAVY (like DT770/880 etc.), because they´re known as neutral and analytical, without any coloration and so from that purposes are used mainly in studios and in engineering or by movie directors, but even by DJ´s e.g.: guys from Kosheen used to use them, so that´s another mark that they don´t lack bass by any mean, no way.
I usually listen at a decent volume.....not too low, not blasting, and I usually have the volume set at about 18 out of 30, so just a bit over 50%. Do you have to raise the volume higher for these compared to another pair of low-ohms phones?
Also, The frequency graph says that they lack in bass. Maybe not for an audiophile, but I've learned that I'm a pretty big basshead. Not in a crappy, distorted way, but the more the better, as long as there isn't distortion and I can still enjoy the rest of the music, too. So is it safe to say that these wouldn't fit that description, even with eq/bass boosting?