The K712 are amazingly comfortable, the pads are soft but strong, they retain shape so the cups don't touch my ears. The AD900x almost succeeded with this by having a very angled driver, but it can still touch slightly. Overall for pad comfort, K712 has an edge for for me. But the angled drivers of the AD900x I think has better presentation though. Toss up?
The bass is surprisingly strong and good with the K712. Both the AD900x and K712 have improved bass for open air headphones that I've owned/used in the past. AD900, HD650 and HD598. The bass is as deep and pronounced as the HD650 but it has similar impact and strength as the AD900 did, so AD900x and K712 both win here. AD900x still has that nice punch that AD900 had but stronger and fuller. K712 while doesn't have quite the punch, I think it goes deeper and fuller. It's a toss up, just depends on what I feel like listening to.
Mids/Vocals, easily AD900x wins here. Female vocals are sublime with AD900x. To me the K712 the mids and vocals are slightly recessed and female voices not as sweet. But the sound is balanced. AD900x slight win here.
Treble, K712 slight edge. AD900x has more sparkle, more fun, but it can diffused at times and is the cause for hearing fatigue. K712 rolls off more to me, it's softer similar to HD598. HD650 still veiled to me. K712 doesn't have hearing fatigue but treble is still detailed. I would even say it's an issue of longer term comfort, so K712 has slight edge here, even though AD900x treble can bring up-lifting music to life more.
Soundstage, K712 slight edge here too. AD900x soundstage is definitely larger, but it's feels artificial at times, which is why I think the treble can become diffused. K712 the soundstage is smaller but it feels more natural and to me, sound separation is better.
Efficiency? On my NFB-12, AD900x I use low gain at around 9 o clock. K712 I use high gain at around 8-9 o clock. Who wins here? I dunno, you decide.
I will not be selling the AD900x, I love them. I'm enjoying the K712 too.
Comparing to K551, well K551 build quality trumps both. Comfort nearly as good, cable horrible. Sound wise it's understated and neutral but detailed. It's good enough for when I'm in public and traveling. That's all I'll say.
One thing would mention, between the 3, after I listen and allow my ears to acclimate to the sound, I can enjoy listening to any three. AKG and Audio Technica made great headphones that my ears enjoy. I hope my observations aren't too polarizing or varying, since I to me all three headphones reproduce sound very well comparatively to my speakers. I don't find any particular sound or element to be way out of sort to distract or annoy me, other than the fatigue on the AD900x after a few hours.
Edited by SSandDigital - 6/15/14 at 4:46pm