Do I need an Amp? Denon D2000 + Xonar DX

You'll be satisfied out of the Xonar.

The Xonar D1/DX have 100Ω output impedance (it is actually a line output only), so using the E11 would improve the bass quality, making it tighter and more controlled. It is not a huge difference, but it might be noticeable. Is it worth $60 and the inconvenience of using the E11 ? I did not try this combination, so I do not know for sure. The Xonar will drive the D2000 to about 104-105dB at the maximum volume (~6 mW power).

Nope just save money to buy new headphones! The D2000's are deliciously efficient despite that big sound

Quote:

Originally Posted by South 

Wow that is pretty damn loud unamped with my Xonar, how do you measure that amount? Do amps only effect the noise(db) it produces or does it tighten up bass and other things as well? I am a major newb at this so bear with me.

According to the specifications, the card can output a maximum voltage of 2 Vrms. I have the PCI version of this card (Xonar D1), and according to my tests the maximum output voltage is 1.94Vrms, with an output impedance of 100.5Ω. It can apparently still be used at the maximum volume with low impedance loads and the distortion stays reasonably low, but the voltage obviously drops. For simplicity, assume 2V and 100Ω. The AH-D2000 has an impedance of 24Ω. That means that the actual maximum voltage on it will be 2 * 24 / (100 + 24) = 0.387 Vrms. That translates to a power of 0.387 * 0.387 / 24 = 6.24 mW. To get the SPL, assume a sensitivity of 90dB/0.073Vrms (source here), and it is calculated as 20 * log10(0.387 / 0.073) + 90 = 104.49dB. According to another source, the sensitivity is actually 1-2 dB better. 105 dB maximum (not average) SPL is actually not that loud compared to what is possible with other headphone/amplifier combinations, but it is enough if you are not listening to something that needs a wide dynamic range.

The main problem is the high output impedance (about 4 times that of the headphone), that does have some effect on the sound quality, mainly in the bass range.

Well, it is best to simply listen to the headphones with the DX only, and decide if it sounds good enough for you in practice. An amplifier with low output impedance would be an improvement, but not a huge improvement. Replacing the headphones is still likely to make a more significant difference. The maximum SPL is only an issue if you cannot get sufficiently loud volume without clipping (which I think can only happen in software, because the card itself does not clip even at 100% volume).

This is interesting, I thought the rule of thumb was to pair headphones with amps that have 1/10 or less of output impedance to their impedance.  So with a headphone with 24 ohm of impedance, you'd want an amp with 2.4 ohm of output impedance or less.  How does this differ?

Quote:

Originally Posted by LithoJazoSphere 

This is interesting, I thought the rule of thumb was to pair headphones with amps that have 1/10 or less of output impedance to their impedance.  So with a headphone with 24 ohm of impedance, you'd want an amp with 2.4 ohm of output impedance or less.  How does this differ?

As I said, it is not quite ideal, and it makes a difference that is enough to be audible, but it will not necessarily sound much worse. Fortunately the AH-D2000 has a relatively flat impedance curve, so there will only be about 1 dB of change in the frequency response. However, the low damping factor may still increase distortion, but without actually having the headphones to test, I cannot tell exactly how much (these graphs seem to show the effect with a similar damping factor, but it is a different headphone).