Headphones to Improve Upon AKG-Q701s

As some of you know I started a thread a week ago regarding Audeze LCD-2s. I have decided to expand my search to include other makes of open back, over ear, planer magnetic headphones at the MSRP of ~$1000. I am currently using a Schiit Vahalla and likely upgrade to the Vahalla 2 which, due to increased gain, will provide a wider range of headphones to be considered. I am considering other models. 

I currently use the AKG Q701s with the Vahalla and very much like the sonics. I am thinking that my quest for a Planar magnetic should result in an improvement over what I have now, specifically in soundstage/imaging. I am not a basshead, preferring quality over quantity in that frequency range, bass to midrange cohesiveness and a smooth top end are most desired.

Please provide suggestions and thoughts regarding those headphones I might consider, as I continue my own research. 

Have you heard the Sennheiser HD 820?   I don't own them but love their sound.  Also....I think the amp driving any earphone contributes mightily to the ultimate performance.
I went from AKG 701Q to Hifiman 400 and the Hifiman kick the snot out of the AKG. It was not even close.
Stringreen, I have not considered the HD 820, however have looked into the HD 800. I do agree that the wide disparity between headphone input/amplifier output impedance and headphone sensitivity/amplifier gain in addition to ones own subjective bias towards what is 'best' certainly begs one to exercise due diligence in matching both components. Given this, I think I need to purchase an amplifier that has a reasonable amount of gain and <3 ohm impedance. 

Raymona, what headphone amplifier have you used with these headphones? Do you happen to recall the sensitivity and impedance for the 400? II have looked into the 400s as a possible entry into the realm of planers at very reasonable price. I need to find how the 400 and 400s differ. 

I have recently purchased the HiFiMan 400s headphones to serve as an introduction to Planer Magnetics. Soundstaging, imaging and resolution has improved over the AKG 701Qs.  I will likely upgrade beyond these at some point. 

Now I am looking for a headphone amplifier what will drive the low impedance loads found in many (most) planer magnetic headphones. Sensitivity is not much of an issue as many can be found that are greater than 95dB sensitive. I would like to hear from those using planer magnetic headphones what amp/planer headphone combination they are using. 
Well, I purchased a Schiit Audio Magni headphone amplifier for a starter. Using it on the low gain setting. Very please with the combination with the 400S. Amazing the audio quality that can be achieved at this price point.