Howdy,

I just wanted to say few words on AKG Q701 and Sennheiser HD598 comparison. I bought Sennheiser one year ago, before that I owned HD558 moded like HD598 hence I am very aware how they sound. Q701 was bought more than 6 months ago.
Gear used:
Schiit Modi DAC
Objective 2 Amp

If you are interested in headphones you may know that generally Sennheiser stuff is expensive more than AKG or Beyerdinamic, but in this case they hitting nearly same price. HD598 often sold as new for $199 current amazon price is $184.21. They can be found like new condition for $150.  AKG Q701 often sold as new same $199, current amazon price is $194.00 they also can be found like new condition for $150 or less. If you have no problem with colors lime green version is sometimes cheaper.

How they sound:
First of all I would like to say this is my experience, my gear, my ears, my music hence take it with grain of salt. However I try to be honest. The main point is that for my ears HD598 is much less in sound quality then Q701.
Things are important for me:
    TIGHT not loosy bass
    Vocal clarity and realism
    Decay or if I can hear extra artifacts
    How much detailed headphone is 

Tonal balance is of course important but I can listen DT880 and HD650/600 and think both have good tonal balance, different but acceptable for me, depends music, mood and needs. On the other hand HE400 or Sennheiser HD700 was more unacceptable for my ears.

Both Q701 and HD598 have rolled off bass they don’t go deep down, impact is also less compared to DT880 or HD650 or HD600/580. Both have wide soundstage, HD598 have more precise/round imagination. Q701 have more separated L R channels, I don’t agree that they sometimes lack centre. This depends gear I guess, never had that problem. Anyway not a big deal, imagine you are in different rooms. Other than that Q701 kick out HD598 in every aspect.
Bass on Q701 is much tight and articulated: drums cello bassoon double bass and other instruments sounds much better articulated and close to real life.  Few examples which demonstrates this difference quite clearly:
Sara K. Turned My Upside Down in album Stockfisch Records - Closer To The Music Vol.1
Sara K Painter- in same album
Patricia barber black magic woman in album companion
Antonio Vivaldi Concerti per Fagotto, I Concerto RV 495 in sol minore Presto
David Benoit, Water To Drink (Agua De Beber) in the album Full Circle
One very important issue I found with HD598 is that it has more harsh splashy upper frequencies which is evident just any vocal recording. For example SSS sound more slow and distorted then Q701- it could be sharp but fast decaying and not as annoying.
Also Q701 have two issues:
    Famous bump at 2K, nothing that EQ cannot fix.
    Uncomfortable headband which can be annoying more than 2 hours for me
HD598 comfort is somewhat OK.
In conclusion I believe Q701 is much better buy if you want more Hi-fi experience compared to blurry and distorted HD598.

Cheers
Lumos