Something that might help though is to see how much actual power in watts or milliwatts you'd need from an amp to achieve a decent volume with the headphones you're using. My math was never very good, but I believe this can be calculated from a headphone's sensitivity in dB/mW using the following equation...
Power = 10^[(Volume - Sensitivity)/10]
The first quantity that we need for this equation is the volume that we want the headphone to achieve without clipping.
The reference volume used in studios for mastering audio content is typically around 85 dB SPL for a -20 dBFS test signal. The transient peaks in music can get louder than this though, and will sometimes approach 0 dBFS. Which means we probably need the amp to be able to handle at least another 20 dB on top of the 85 without clipping. Or around 105 dB in total, to achieve something close to reference levels. Audiophiles tend to like even a bit more headroom than that though, just to be on the safe side. So we'll add another 10 dB, to bring that up to 115 dB.
These are somewhat arbitrary values, which may be a little on the higher side. But they are in line with the levels used for THX certified gear. And 115 dB ensures that we have ample volume for quieter content. (Normal listening levels would tend to be lower than this though, probably peaking at only about 95 dB on the transient peaks.)
Next we need the sensitivity of the headphone in milliwatts.
When comparing a headphone's sensitivity (or efficiency), you need to be sure you're looking at the same spec. Because some manufacturers will use dB/mW for this, and others will use dB/V. (One can be derived from the other though, so that apples can be compared to apples... hopefully.) Etymotic lists the
ER4XR as 98 dB/mW. And the
ER2XR as 96 dB/mW. If those values are correct, then the ER4XR probably needs about 50 mW of power to achieve our target volume of 115 dB SPL. And the ER2XR needs about 80 mW...
Power = 10^[(115-98)/10] = 10^1.7 = 50.12 mW
Power = 10^[(115-96)/10] = 10^1.9 = 79.43 mW
The ER2XR needs a little more power than the ER4XR to achieve the same 115 dB reference volume because it is slightly less sensitive.
To see whether our amp can really deliver the goods though on the above, we also need the impedance of the headphones. Because the amp's power output varies depending on the load impedance. The headphone is the load in this case. And Etymotic shows the ER4XR as having a 45 ohm impedance. And the ER2XR at a mere 15 ohms.
The graphs that Doodski posted above show the Topping amp's maximum power before clipping (or maxing out of the volume control?) for two different load impedances: 300 ohms and 32 ohms. Since our headphones are both at the lower end of the impedance range, the 32 ohm graph is the closest. And with a 32 ohm load, the maximum power on the Topping is 119 mW in its Low Gain mode, and 1522 mW in the High Gain mode! With the 45 ohm load on the ER4XR, the max power will be slightly less than these values. And with the 15 ohm load on the ER2XR, it should be more.
Assuming the above figures are all more or less correct, this suggests to me that the
Low Gain mode on the amp should have sufficient power to drive either earphone to reference volumes without clipping. The only way we can tell that though is by looking at both the headphone's sensitivity, and also its impedance. So both come into play here.
The High Gain mode on the Topping appears to have appreciably more power than necessary to drive either earphone to ref volume. Which might seem like a good thing at first. To use that mode at a comfortable listening level though on the headphones in question, you'd need to drop the volume down quite a bit, which could have the potential for making noise more audible.
