I really don't understand you Fiio. X7 price increase, moreover X7 amp has bad crosstalk. DAP and dac/amp are replaced even faster than before, and now, you remove fiio e12a. It's not fair... Sure the sound of an amp is subjective to everyone. We haven't same system, same cans etc... However, i am sure you can ask any electronic engineer, there are some objective specifications of a good amp like low distortion, SNR etc...
That's why this new amp shouldn't replace the e12a model:
-The battery life is really decrease, so why don't let people choose the one they want ? Lower voltage but better battery life, or High voltage and lower battery life, or better battery life on a5 as options...
-The crosstalk is one more time ignored... I have e12 and e12a on my own, i can really hear a huge stereo difference because the crosstlak is 20db better on e12a, and a5 haven't this great crosstalk. It's really important because loud speakers on headphones are very close...
-distortion in low gain not really important at this point, 0.001% difference isn't huge, however in high gain, 0.01% THD a bit high because headphones distortion and others settings add a lot distortion, but that's great result compare too some other manufacturers.
Now let's talk about voltage required for headphones. To be easy to understand, a low impedance headphones need high current, whereas high impedance ones need high voltage. It's deduce form Ohm law: U = Z x I and the power P = U x I. However, except if you want to be dull as fast as possible..., all good amp have enough of both to reach high dynamic. You don't listen for a 120db peak of a drum... Believe me, your ears will give up before you can hear it... The difference you hear from a high voltage amp, is not about dynamic max level, we don't listen enough loud for, but about rise time. (fastest time possible by the amp to change from low voltage value to high one, generally 10% to 90%) It's all about it. The more voltage you get, The less rise time is. And it is really important, because if the sound level change too fast, your amp can't reproduce it, and you lose some sound details. You need more and more short rise time when frequency is high, because the more high is the frequency, the less time you have to change sound level. That's why some amps seems sound warmth, neutral, sharp etc... It's all about taste.
about the e12, almost all specifications seems better overall, maybe not for distortion graph in high gain, but i am pretty sure fiio e12 graph was only low gain, that's why distortion lower than a5 in high gain, and voltage peak is a bit under e12 15.5V. So, it's ok to replace the e12 only.
So lot manufacturers know how to do a good amp, but they don't care about it, the goal is to sell not to satisfied the customer...
There are easy rules to know what is a good amp.
However i understand fiio want make pretty cheap amp for people. I am sure they can make a portable amp with really more voltage, more battery life as you can see on smartohones, high 100db crosstalk and so on, if it will be really more expensive. Really expect it.