I have owned a pair of AKG K501 for years. I never really liked them as I found them to have a magical midrange, their bass was lacking. I just got a cetrance DAC/headphone amplifier, and I changed my opinion. They now do everything I want. Wondering whether any of the newer AKG 7XX )701,701, 712) will give significant improvement? I also have Sennheirser 700  and Shure 1541.
I know this is the speaker forum, but there is not a headphone forum.
listen to Audeze.  They knocked my socks off.  "LCD-2" for affordable high end.  "Sine" for sealed travel or work headphones.  I bought the Sine to use at my office desk and on airplanes.  I will buy the LCD-2 next...

The sennheiser 600s are a reference for many.
Had 501's, sold them. Had 701's, sold them. Own HD600's. Not for sale