I would call it very neutral, from 500Hz upwards it completely neutral, only thing that may make them not neutral is the bass, but the bass still feels very neutral while listening, to be honest the Q701 peak a lot more, which is not neutral either, both have their signature and both are neutral except for the range they boos, X1 boosts bass from 70Hz to 500Hz and the AKG Q701 boosts 1700Hz to 2100Hz, but there is no completely neutral headphone, there is not one headphone that doesnt boost its own signature, but both the AKG and the X1 are neutral overall and with that sound very natural which to me is very important, thats why i prefer HD600,X1 and K702 over the DT990 or T90 or Beats or Deneon or anything out there besides Audeze, Beyerdynamic DT880 / T1... Really both X1 and AKG K702 are absolutely neutral besied they signature^^ Those who prefer a tad of warmth go X1, those who prefer a tad brightness and close vocals go K702 and the HD600 is kind of a mix between them but suffers from smaller soundstage. Thing is the difference between thos 3 headphoens is so minor to be honest, anyone whos like OMG X1 is so bassy but like the K702 is a bias liar in my opinion, because they really dont differ that much, X1 has a tad more punch and the K702 close vocals, the K702 is in danger of being sibilant, the X1 or sound a bit too warm for some, but in the end the difference truly is minor
DT990 for example is so bright its not neutral anymore, no naturalness at all in my opinion, HD650 is truly a tad to warm for me, but still its natural and quite enjoyable