For the specs, look for:
- Low rated THD, though it's rarely if ever measured as worst case. You want to aim for 0.05% THD as worst case, so look for 0.01% rated or thereabouts to be safe
- Low noise level (a high negative number, preferably -90dB or lower)
- High SNR, preferably around or above the noise level
- Output impedance less than 2 ohms. If it's not published and you can't find it elsewhere, ask here. If no one knows, don't buy the amp. Your headphones have low impedance, so this can be important.
- Output power into various loads. Not too important for the headphones you have, but if the output impedance isn't published the output power will often come in handy to try to determine it. It may also be important in the future, when you buy new headphones, but it's really not at all as important as everyone thinks it is.
You're probably going to want to avoid the EF2A. It uses tubes in its input stage, and they're inherently high in THD (though it's a pretty good
kind to THD). I don't think Hifiman publishes much in the way of specs, either. The ones they do publish aren't very promising. 95dB SNR isn't great but not bad. The drop from 6.2V at 150 ohms down to 3V at 32 ohms
suggests that its output impedance is quite high.
The Fiio E7 is a quality product, and you might want to consider it alongside the E10. The same blog that showed how good the E7 is
might review the E10 later in the year. You have time to wait, the Macbook's headphone jack is probably decent at the least and unless you're running out of volume there's no need to panic. I don't see why a new DAC or amp will be a significant upgrade for you.