I have followed this forum for years and found lots of useful information about headphones and related stuff. Based on what I read here I bought the HD650, over four years ago. Couple of days ago I purchased the AKG K701 to accompany the HD650. I have been comparing the headphones since. Now I want to give something back to the wonderful community here. I presume there are many forum members who are speculating which one of these headphones to buy. This is my small contribution to the discussion. I will try explain what are the differences with these headphones, in my opinion :) Of course the best option is to try both headphones before buying.
I have not done any burn-in with the K701 because I do not believe in it (except for deformation of the ear pads during use, which can effect the bass performance slightly). I know many people say these cans require hundreds of hours of burn-in. The K701s have about 15 hours on them at the moment. Actually, I contacted AKG via their email-hotline and they stated there is no evidence of burn-in. I do believe in psychological burn-in. I mean our branes will adapt to the sound the headphones are producing. Before buying the new pair I am now using, I had a demo K701 (from a local dealer) for home listening test for two days. My brains burned in then. The new cans sound exactly the same to me. But if I find any sound signature changes with time I will make an update.
Amps, DACs and cables
I used to have a hybrid tube headphone amp with the HD650s but when I got the Benchmark DAC1 USB in 2008, I sold it. I really like the HD650s with the Benchmark. I also have a Nuforce Icon HDP, which I use as my main DAC/amp for headphones. I use Benchmark nowadays mainly with my speaker system. Because of this, it is physically situated in such an awkward place for plugging in headphones that I rarely use it for this purpose. In my opinion Benchmark is a bit better DAC/amp than HDP but differences are small, with both headphones. From my experience, I do not believe there are - significant - differences with amps and DACs when a certain level has been reached, and HDP reaches that level. HDP drives both headphones adequately.
Couple of days ago I had an "upgrade" Cardas the cable for the HD650 for a home listening test. After reading huge amounts of positive impressions on this cable, I was really expecting some results. First impressions were good, but after some ABX testing with the stock cable, I had to admit it to myself. I heard no difference between the cables. So I am a happy HD650 owner with the stock cable. I have never tried balanced setup, so I can say nothing about that.
Yes, I know this is my opinion and not many other forum members will agree :)
Built and comfort
I like the way K701 looks like when it sits on my desk on its own headphone stand, next to my Macbook Pro. Though I must say, when I am wearing the K701s and catch my reflection on a glass coated picture frame next to my chair, I smile. These phones are quite goofy looking on my head :) Not necessarily a bad thing. HD650 looks are not nearly as flashy, but there is nothing wrong them either.
Paint is not falling off my HD650 but I have heard many who are having problems with this. K701s have soft flimsy plastic parts on both ends of the inside of the headband. Overall build quality is good with both cans.
Both headphones are fairly comfortable. When I put HD650s on my head I feel gentle but firm clamping around my ears. After couple of hours, this clamping usually starts to irritate me a bit. However, the clamping also gives an impression of certain authority and control.
Though K701s looks more massive than HD650s, there is no real difference in weight. Actually K701 feel very light relative to its size. I feel the K701s are more comfortable for extended listening sessions. I don't mind the bumps inside the headband. Though after HD650s the K701 feel quite flimsy and I have to sometimes be careful not to drop them when I am jamming with my tunes. With the HD650s I feel like I could do backflips with no danger of them falling off :) When I carefully go to lying on my bed with the K701 they would drop if I didn't hold on to them. Also, because the K701s "cups" are so big and round, they are pushed upwards a bit when I'm lying on my back with a pillow under my head. Moreover, because of the automatic headband adjustment, I sometimes have trouble positioning the K701s on my head. That irritates me, but the feeling may go away when I get more accustomed with the cans.
The HD650 feels more Hi-Fi on my head, and guess what, I like that feeling :)
K701 is famous for large soundstage. Actually this was my initial interest for these cans. I wanted larger soundstage than what my HD650 was delivering, especially for classical music. K701 does not disappoint, it surely deserves all the compliments. But I would not say that the K701s imaging is more accurate. HD650s soundstage is smaller but it is very well defined.
The K701 creates a lot of "air" around the instruments. This makes many well produced recordings shine, but there is a down-side though. Because of this airy appearance, there will always be a certain distance to music. The HD650 feels much more involved. This was the biggest surprise to me after reading reviews and impressions from both cans. Therefore, I feel there are genres that are not suited for the K701. The headphone is not able to deliver the raw power and energy that rock, punk and metal music are about. I believe this is part of the reason why many people call these headphones thin-sounding. "Plugged-in" music is meant to be heard without any acoustics in-between. Sounds that are meant to be "on your face" will never be there with the K701. This is something that amps/dacs/cables have no effect on, in my opinion.
Classical music is practically always performed in an acoustically "correct" room. So, it does not make things worse if the headphone creates some "artificial" space in to the mix. Actually, I think K701 proves that, with right circumstances, beautiful things can happen. I do understand people who think that K701 soundstage is unnatural, but I don't think it's the whole truth.
Bass, mids and treble
I feel there is very little difference with the mids, no complaints. Bass and treble performance is totally another story.
Treble on the HD650 is famously rolled off. I believe this is something that Sennheiser chose to sacrifice over non-fatiguing listening experience. AKGs approach is more forward. K701 sounds quite bright and therefore more fatiguing than the HD650. In my opinion, good thing about bright cans is that I don't even want to crank up the volume during listening. Of course, too bright headphones can be too piercing and unlistenable, but the K701s are not too bright for me. The laid-back sound of the HD650 can be dangerous because of potentially too high listening levels. Especially with rock and punk music I feel the need to crank up the volume. So the HD650s are not the best cans for rock music, either :)
Bass performance is, like for many people, the weakest link on the HD650s performance. There is a bit too much of bass, and it is quite poorly controlled. I really wished that cable upgrade would have given more quality bass, but no such luck. I believe bass is an area where an amp can make a small difference. The bass on the K701 is well controlled but a bit lacking for my taste. In addition to the point I made before, I believe this is why K701 can feel a bit thin-sounding.
I wish these cans had the bass similar to my closed portables the Sennheiser HD25-1 II. There is some yummy well controlled bass. Actually I feel the need to put those cans on...wait...oh yes...mmmm...that's what I am talking about ;)
I agree with most people, the K701 is a neutral sounding headphone. HD650 is warm and a bit bass heavy.
There is no doubt, K701 delivers more detail. That does not mean HD650 lacks detail. I don't think more detail is always better, when we are talking about enjoyable listening experience. With the amount of detail HD650 puts out, it feels more refined, calm and sophisticated. K701 feels more edgy and harsh. It is interesting to hear the soft details K701 puts out. But in the long run, I like the Sennheiser approach to details more.
I will go through some music where I feel the K701 shines on like a crazy diamond!
Pink Floyd - Shine on You Crazy diamond
Simply beautiful through the K701.
Rajaton - Jos sanot
Well recorded A cappella. Sorry about the bad youtube quality but I still think it will show the point I am trying to make.
Joan Baez - In Concert (1963)
Soft folk rock. I like the production stereo effect used here. Her voice is moved around the soundstage with skill. K701s wide soundstage emphasizes it better than HD650s.
Steely Dan - Aja (album)
Some laid-back and well recorded jazz-influenced rock. Sounds outstanding on the K701.
Claude Debussy - New York International String Quartet - String Quartet in G minor (1990)
Chamber works are a strong point of the K701. In a word, amazing. Better than the HD650 performance, which already was perfection. So I found another level of perfection! :) K701 feels more open sounding. I guess this could have something to do with the HD650s rolled-off highs.
Glenn Gould - Goldberg variations (1981 version)
How about some stereo solo piano for the K701? Oh my, oh yes! Wider and airier soundstage works wonders. I also like the fact that Gould's famous sing-along is much more clearly heard with the K701.
Joni Mitchell - Blue (album)
Compared to the HD650 Mitchell's overly naked voice is even more naked with the K701. Sad, beautiful and life-affirming.
The Beatles - Because
Soft harmony vocal performance taken to a next level by the K701.
Stuff where the K701 makes me feel so suicidal, even hate my rock and roll!
The Beatles - Yer Blues
A piece that was recorded in an small kitchen closet, to make it raw and energized, at Abbey Road while smoking some pot. This is were HD650s shine with some crazy deep but simple bass.
K701 performance? Absolutely awful. Oh no. Fail.
Ramones - Blitzkrieg Bop
Punk and K701? Stay away! Far away.
Nirvana - Serve the Servants
What did I tell you? No!
HD650 performance is quite nice, but the need to crank up the volume is, again, imminent.
Björk - Bacherolette
Not too bad on the K701 but Björk's heart-aching vocal performance is felt much more intimately with the HD650.
Ok, I hope you realize there was a bit of dramatization involved on those last few comments :)
I think that most larger classical pieces like concertos and symphonies will sound a bit better with the HD650. Better defined soundstage, deeper bass and less-fatiguing sound are the main reason. K701s wide soundstage is not so well defined therefore many instruments at once sound a bit messy. Though I am not sure about this yet. I have to listen to more symphonies.
If I only listened to rock, punk or metal I would not buy either headphone. There are lots of other cans better suited for that purpose. I like both headphones a lot but if I had to choose only one, it would be the HD650. It's a better all-rounder.
The K701 is an outstanding headphone when it is used with proper music. Most classical performances and some other laid-back stuff really stand out positively with these headphones.
Actually, I fell in love with my HD650s again after purchasing the K701. It was the right move to spice things up. Having new enjoyable toys tend to have that kind of effect in a long relationship. And yes, I am aware what that sounds like ;)
Because the HD650 cable upgrade did not do anything for me, I used that money for something else. I just ordered Alessandro MS-2 headphones for rock music. I have never heard them, but I hope they are similar to Grado SR325is, which I have heard and liked a lot. Grados are just so insanely expensive here in Finland. SR325is is priced at 400€ (about 565$). Alessandro MS-2 will cost with customs only about 270€.
Before my AKG K701s I would have never guessed I could be so picky about what kind of music I would use with different headphones. That's funny.
English is not my mother tongue so I apologize if it is difficult to understand what I am saying :-)