Hmmm for the purposes of being fair... I will only compare them single-ended. The HD650 balanced are a different thing altogether... and I have not had opportunity to try the Annies balanced (yet... they may get modded in a year or so? )
Now I have a very VERY big head but I find both headphones to be very comfortable. I'm sure most of you reading here are extremely familiar with our beloved HD 650s. So I will move on. The Annies of course have the signature AKG headband that self-adjusts with the elastic cords. This headband design is actually very well implemented and convenient for when you share with other people to listen... no need to fiddle with adjustments, just stick them on and they will be the right size. The Annie clamp lighter than the 650s so I had no issue there, no need to stretch them. As well, the Annies earcups are a very very marshmellowy like memory foam... a complete joy to wear they are super soft and comfy and will mold to your head to provide a semi-seal. But if you're in a hot room/summer then I could imagine it getting too warm (I'm in Canada though... so no problem ). Overall build quality is very nice with good sturdy fit and finish... the Annies headband is genuine leather, and the gunmetal titan finish with blue trim is gorgeous! Detachable cable is also well made with a good plug... the mini-XLR goes in with a very satisfying click (not as scary as the first time i had to pull the HD650 connectors)... sadly, the XLR is 3-pin... not 4-pin, meaning it will require modification before it can go balanced.
The Annies win out here... quite easily as a whole. The detailing, shine and shimmer are definitely more well defined here. Cymbals have more sparkle, violins have more bite, acoustic guitars have more of the live (yourself playing) sound... which I like as a humble acoustic guitar player. So for albums like Eric Clapton's Unplugged... I generally will prefer the Annies over the HD 650. Trebles in the Annies are not particularly harsh, and only in some more treble happy songs/albums will I find them fatiguing. The HD650 of course as all others here agree... have a nicely rolled off treble allowing for some seriously nice easy listening and as I'm treble sensitive, I generally appreciate this. However, the HD650 are not "lacking" treble... it can still be pretty bright for some songs, but just much more forgiving so sometimes it can seem to have less "shimmer/sparkle" than brighter cans.
Bottom Line: Annies have better treble overall
Interestingly, it is here that I find the Annies to share some character that is reminiscent of the HD650s. The mids could be considered fairly neutral with just a slight tint to the darker side (compared to the brighter Q701). The sound is well balanced in this region... vocals have superb detailing and tone, piano keys have a good life-like timbre. Overall smooth transitions and relatively more forward sound with the vocals. The HD650 has slightly smoother mids, sound is slightly brought back, and you don't get as much of that clarity in the micro-detail that the Annies show... it's not to say that the detail isn't there... it's just not demanding your attention like the Annies... you have to subtly look for them (like coffee or wine tasting).
Bottom Line: I'd consider this a tie between the two... it's quite close IMHO
Now all of you KNOW I had to make a separate section for this . This is where ALL THE MAGIC HAPPENS!!! Truly, the region that makes the HD650 sing... providing that ultra buttery sound that gently floats you up in a euphonious wave of chocolate sounds. Have yet to hear any other headphone perform quite like the HD650s in this region... yes you may criticize and say this region is coloured... and you would not be wrong! But it is coloured oh so lusciously!!! Some hate the HD650s because of this... but for many others... it's simply one word: LOVE. Now... before I completely forget what I am doing here, back to the Annies: They actually do have some characteristic warmth in this region too which is why some others here in the forum will agree with me that the Annies do remind us of the HD 650. However, they certainly do not quite reach the level of colouration that is found in the Senns... the Annies are a bit more controlled... providing some smooth lubrication to the sound but not quite the richness of the HD650s. Some may actually prefer this if they find the HD650s too overly smooth (no real such thing in my book tho ).
Bottom Line: Do I even need to tell you???
Here is where some may be deceived by initial A/B comparisons for the Annies and HD650. Depending on the track/music it is easy to say that the HD650 wins out for bass... mostly because it's just more obvious and easy to notice. BUT, if you test a variety of tracks and pay attention, then something quickly becomes apparent. Both cans are very capable of bass... but they do it slightly differently. The HD650 have a nice enveloping full sounding bass... it is well textured and has generous quantity without being boomy, but can be considered "slow" depending on the music/preferences. The Annies on the other hand, seem to have a less enveloping bass with that warmth that wraps around you... rather, it seems to have better lower extension with a nice visceral punch. So it's quick and snappy with good tight control and will come and go when needed... not staying long to hang out and chill. One way I like to see it... is the Annie delivers bass like a quick jab to the body... whereas the HD650 is more like a swinging haymaker.... more weight overall... but not as quick and controlled.
Bottom Line: A matter of preference IMO... HD650 has a bit more quantity and enveloping warmth... Annies have tighter lower punch
Soundstage: (again I must remind you it's through single ended... balanced HD650 is a different level and easily trumps HD650+Annies running SE)
The HD650s image fairly well... but overall the soundstage is a bit smaller, and the instruments can seem a bit more crowded together. Here the Annies win out with a greater sense of air... better precision imaging and a very wide soundstage. Oddly enough though, the Annies have a wide soundstage but not a very deep one (I had heard how these had amazing soundstage and had thought they would provide more depth).
Bottom Line: Annies win out with a wider soundstage, greater sense of air to the sound and more precise imaging.
All in all, I really like the Annie because it's similar enough to the HD650 that it doens't wander far from my tastes, but yet it is lively enough with it's own character to complement the Senns well. If someone destroyed my beloved Senns and got rid of the rest of them (I would hunt them down KILL them later) the Annies would likely be my next candidate to live with after. All in all the Annie is a very solid headphone that I am proud to own.
Haha, I guess this wasn't so quick, hope this helps