[Me x3]

   
The 62-Ohm AKG 700 series is considered to need as much power (if not a little more) then the 250-Ohm Beyer headphones.

 
To be more precise:
 
1- AKG K712 needs almost the same voltage swing as Beyerdynamic DT880 250 Ohm.
2- AKG K712 needs more than twice the power needed for DT880 250 Ohm
 
DT880 250 Ohm is easier to drive, you can plug them almost everywhere and they'll sound just fine.
AKGs on the other hand can produce high distortion when plugged into amps with very little current capability.
 
That said, Fiio E10k can drive both just fine. Might not be the last word in amplification for these headphones, but it works, and you get great sound quality.
 
--------------------------------------------
I wouldn't say that T90 spanks the K712.
 
Both are good headphones with slightly different sound signature. The T90 is probably more transparent and detailed, but can be too bright for some.
On the other hand, I'm sure a lot of people would argue that the K712 is the more natural sounding headphone in terms of tonality.
 
As KG JAG stated, the preferred sound signature plays a very important role here.
--------------------------------------------
 
Back on topic, both (DT880 250 Ohm and K712) are very nice headphones.
The Beyer is brighter, if you are really sensitive to treble, sibilance and so, stay away.
The AKG is more forward in the upper midrange, but smoother in the treble region.
Different flavours, you know...

 

[Darkseth]

Even worse: Some said, it needs more power, than the 250 Ohm DT-series.
The T (Tesla) series is MUCH more efficient.

 

[Me x3]

  Even worse: Some said, it needs more power, than the 250 Ohm DT-series.
The T (Tesla) series is MUCH more efficient.

 
Are you referring to K712 vs DT880 250?
It's not something someone says, it's objective fact based on measurement. Both headphones need similar voltage. The AKG is only 67 Ohm so it drains much more current, which means it needs much more power.
 
Tesla T90 is easier to drive than DT880 250 Ohm.
Tesla T90 is very similar to the Sennheiser HD600 in that regard.
 
HD650 is even easier than T90/HD600