I own the AKG K240 DF Studio monitor headphones, and I have been using them for more than 10 years. 
Assuming that, after so long period of time, the headphones sound quality should have improved, I decided to make a research on the market in order to find a substitute of my old headphones. 
I took into account technical specifications, reviews and prices, as well as the availability of some brands in the city where I live. 
Discarding other brands, I considered choosing between Sennheiser and AKG. Sennheiser was generally more available than AKG in all the shops I visited. But I have always used the AKG and I have been very happy with them, so, eventually I decided to keep on buying AKG headphones. 
The present AKG top model is K701 one, but the average price I found in the shops was about 350 €, which was too expensive for my budget. 
Fortunately I found a person who was selling his second hand headphones of this type at an affordable price for me. 
 
I was impressed by the incredible sound quality of the K701. The music is so detailed, so clear, so balanced and the frequency response is so extended that I doubt anybody can find anything better, at least in this price range. I have heard that there are headphones that cost even five times the price of AKG’s, but this is another world and, anyway, I have neither the chance to listen to them nor I could afford them by no means. 
I have never heard the instruments so separated and exactly situated in the sound space as I have heard with this headphones. I realised that it was time to move away my old headphones. Moreover, I have read that the sound quality of the headphones improves after 300 hours of working, so I expect a better performance after some months, if possible. 
 
If I had to change something of the headphones, I would come back to the leather pads of my old K240 DF, instead of the velour ones. But this is due to a personal problem that I suffer, being allergic to some kind of fabric. This is not AKG’s fault. 
 
I use a Cambridge Audio 340 SE amplifier to drive them. The CD player is a Sony CDP-X8920 Q series. Although I am aware that this is not the best equipment that an audiophile would desire, I think it is enough to appreciate the great sound delivered by the headphones. 
I have clearly noticed the great differences of record quality among the CD’s I own.