Wow!! what a great and friendly community. I've never thought of such a detailed and kind replies.
Thank you again Digital-Pride & IEMCrazy.
Dear IEMCrazy, yeah you are right, I'm a little bit over-expectant. I want a headphone that has all of qualities that I'm longing for and as you said it is impossible. and about your suggestions:
HD 650 :
Yeah, they are great. But as I remember when I checked them last year and compared them to my friend's HD 600's, I liked HD 600 more. But overally their (HD600 & HD650) treble response seemed to me a little bit unnatural.
Nothing wrong with preferring HD600, they're generally viewed as equals. HD650 is a bit of a technical edge, but basically they're the same headphone with two different voices. They both scale almost equally well with source upgrades as well. HD650 is "polite" a bit warm, and mids-boosted. HD600 is more linear but still with the smooth sound of the series. HD600 was designed originally for classical mastering in the studio to give you an idea of its linearity. It won't have substantial bass either but less of a midbass hump than HD650. Treble response is more rolled off on HD650 than HD600. But one thing is true of pretty much all high end Sennheisers: They are amp picky. From HD595 through HD800, they're pickier than most. Choose poorly and they sound bad. Choose acceptably and they sound average. Choose well and they sound extraordinary. Senn is definitely rewarding of those who take the time to build their system, but for that reason, isn't always the most obvious choice. If not wanting to go through research/purchase of the rest of the signal chain, HE-400 comes out ahead. Not that source upgrades don't help those too. But HE-400 more easily hits a really good level. HD650/600 takes more work to hit that level, but when it does, it can outshine HE-400. It doesn't need fortunes in gear always. I ironically found I liked my HD650s more on my $40-60 Fiio E11 than I did on my $300 Headroom Micro. But I like them on my $450 Lyr best of all. It's more about patience and research (and talking to folks here) than sheer expense to find a good balance for Senns.
AKG K70x/Q70x:
Some say Q701 has better bass & mids response. Is is true? and a dead-pan flat response is a negative point or a positive one?
HD650 is one of the kings of mids. Hard to beat there. It has an accented midrange, as does HE500. All planars should have smooth, detailed mids by nature. I haven't tried Q701, though it's possible my newer manufacture K702 use the updated drivers, I'm not sure. Officially the drivers haven't changed, but too many people noticed a difference, and headphone makers love making changes they later deny having made. AKG sound is very different, being so flat. But once you get used to it it gets kind of addictive. It's neither positive nor negative, but simply one possible sound signature to choose. Other flat/analytical headphones are Etymotic ER4 IEMs, HD800 (though it has a treble bump), and a variety of studio monitors. Some folks love analytical, some don't. For me I wouldn't choose it as first choice, but it's nice to have in the lineup.
DT 880:
Can I make them less treble-happy with amping?
You can always EQ down the dreaded 4kHz treble spike that accentuates the sparkle. Some warm tubes in an OTL amp would probably help too. Parametric EQ is ideal though, and if you're not using PC as a source, those beasts are expensive....
K550:
I've read Mike's review. It seems a little bit exaggerated. is it that good really?
Coloration is always personal choice, and closed backs are always more colored. Usually anyway. wje has a thread going for modding the K550 if you're into that. But a lot of folks seem to like it stock. If youre going for neutral bass, I'm not sure it's the optimum, but it is a studio headphone...
D5000:
From what I've read here, It seems HE-400 is a better choice than D5000. However, I'm not certain.
I personally like HE-400 better. D5000 is still less amp dependent, and it has both the best bass this side of LCD-2, and it has more treble sparkle than HE-400. I have D5k and like it, but I do like HE-400 better for most genres. The Denons win for electronic music (or music featuring a fair measure of electronic bass), as well as for raw wood acoustic instruments like a guitar quartet or solo violin where the wood cup sounds very natural. Mids are very good, but the planar sound is very addictive.
HE-400 comes very close to HD650 for me, and HD650 only bested it once I added a silver cable and chose some nice warm tubes, making it the far more expensive headphone (though the tubes benefit HE-400 as well.)
HE-400:
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When you say It doesn't have a huge soundstage, you mean in comparison to K70x/Q70x that people say have monstrous soundstage or in comparison to for example DT880? and how much its soundstage is smaller than them?
Planars in general lack the soundstage of many dynamics in its category. Thus while LCD-2 probably has better soundstage than DT880, it's in a different tier, LCD-2 can't beat the soundstage of T1 or HD800. HE-400 has been said to have a little less soundstage than HE-500, but I suspect that has to do with the stock pads which can be swapped for $10 to the same velour. K70x has a monstrously wide soundstage, but not as much depth as you'd expect. It's very nicely wide, but without exceptional imaging/placement compared to HD650. HE-400 on the other hand has great instrument separation but the soundstage is generally relatively close to you, and placement also doesn't quite match a well tuned HD650 setup. (Comparatively, HD650 has a "three blob effect" with one left, one right, one in the middle of your head when it's not being run on a well tuned setup.)
If stage width is key, AKG wins. If depth/placement is key, Beyer or Denon probably wins. But planars like HE-400 are unique. It's not the best soundstage, but you tend not to notice as much, due to how natural the sound is. And they can also produce the famed "3d sound" similar to 5.1 recordings with rear speakers or Dolby Pro Logic. Not holographic as is sometimes described, but 3D. With HE-400 I don't sense the same vast performance I do with HD650 or K702, and yet the performance is more phisically present at times. Planars can be hard to explain. I imagine it has to do with the sheer diameter of the diaphragm.
Thanks