[Me x3]

  If you do the math, the Amp would need a very high output impedance to create this effect. An Amp with a 10 Ohm output impedance would only reduce the level at a 300 Ohm load by 0.282 dBV, If the impedance goes up to 600 Ohms then it would reduce the level by 0.140 dBV. That's a difference of 0.142 dBV which is inaudible. Change the Amp's output impedance from 10 Ohms to 100 Ohms and the difference from the same two load points becomes 1.16 dBV, again not very significant. IMO don't buy an Amp with an output impedance much greater than 100 Ohms. Most Amps have a far lower output impedance, so I wouldn't lose sleep about my HD600.

I think 100 Ohm might yield audible results, it's a wide region between 45Hz and 250Hz where you get a slight boost (0.5dB < boost < = 1dB). Very subtle, that's granted.
That said, you can find higher than that. I had a modern Yamaha A-S500 integrated stereo amplifier and the headphone output was 470 Ohm. Great speaker amp for the money, btw.
 
On top of that, high levels of second order distortion is the bass increase the sense of bass quantity.
(40Hz + nothing) sounds different than (40Hz + audible 80Hz)

 

[StanD]

Yeah my amp has high output impedence

I don't think so. Are't these the two Amps that you mentioned?
Project Polaris: Output Resistance: Selectable 0.1, 35 or 120Ohm
Lake People G109: solid state (tends to be very low output impedance) They say "highest transmission quality for low- , medium- and high-impedance headphones."

 

[StanD]

 
I think 100 Ohm might yield audible results, it's a wide region between 45Hz and 250Hz where you get a slight boost (0.5dB < boost < = 1dB). Very subtle, that's granted.
That said, you can find higher than that. I had a modern Yamaha A-S500 integrated stereo amplifier and the headphone output was 470 Ohm. Great speaker amp for the money, btw
 
On top of that, high levels of second order distortion is the bass increase the sense of bass quantity.
(40Hz + nothing) sounds different than (40Hz + audible 80Hz)

Your Yamaha probably has an output series resistor for the headphone output. Use the speaker taps instead.
It takes 6 dBV to double the gain, decibels are not linear so 1 dB is very small, 0.5 is probably not even perceptible. Headphone Amps tend to have low output impedance, so don't buy one with a high output impedance, I wouldn't. An amp that has that much 2nd order distortion to be JDD is a terirble amp, I would never buy something like that.

 

[danmlr]

  It used to be, but it's quite long in the tooth now and even older than the HD600, which isn't exactly a young headphone itself. For my personal preferences, the SHP9500 is just about the best headphone you could buy under $100. It's a little lacking in bass and perhaps a touch too energetic in the treble and upper mids, but overall, it's the best headphone I've heard in that price range, open or closed.

I'm looking forward to it. I've had mixed results to say the least, with the recommended headphones on head fi. I felt like I did score BIG TIME however, by going with the X5 which has received a lot of justifiable hype here, and with the VE MP, which is outstanding at the paltry price of $7 to $13 shipped. I wasn't as fond of the X3ii which has a great design, but rolled off treble and therefore mediocre detail. 
 
I'll keep my fingers crossed with the philips.

 

[Me x3]

  Your Yamaha probably has an output series resistor for the headphone output. Use the speaker taps instead.
It takes 6 dBV to double the gain, decibels are not linear so 1 dB is very small, 0.5 is probably not even perceptible. Headphone Amps tend to have low output impedance, so don't buy one with a high output impedance, I wouldn't. An amp that has that much 2nd order distortion to be JDD is a terirble amp, I would never buy something like that.

I don't have the Yamaha anymore, but it sounded quite well indeed. In particular with Beyerdynamic DT880 250 which has a less pronounced impedance bump in the mid bass region than the Sennheiser HD6x0. DT880 is also less warm so the added warmth can be a good thing, depending on the recording for sure.
 
I know 1dB is small, but what I've said is that a small 1dB bump ranging from 45Hz to 55Hz is much harder to hear, than a broader 1dB bump from 45Hz to 250Hz. Even then, it's small, but probably audible in the long run.
 
All my amplifiers have low output impedance, but perceived sound quality is much more complex than just a flat amplifier, so I would say buy the amp that make you enjoy your music the most.
 
Not sure what jdd means, but HD600 has quite high second order distortion in the bass, much higher than most amplifiers, and most people like HD600's bass.
 
HD800 has less second order distortion in the bass than HD800S, but most people prefer HD800S bass. (I do keep my HD800)
 
Pure objectivity for the sake of objectivity is a fine excercise. But enjoying the music is most of the times unrelated.
Mainly due to the vast disparity among recordings.

 

[Midgetguy]

 
I'm looking forward to it. I've had mixed results to say the least, with the recommended headphones on head fi. I felt like I did score BIG TIME however, by going with the X5 which has received a lot of justifiable hype here, and with the VE MP, which is outstanding at the paltry price of $7 to $13 shipped. I wasn't as fond of the X3ii which has a great design, but rolled off treble and therefore mediocre detail. 
 
I'll keep my fingers crossed with the philips.

I wish you luck. It's unfortunate that you won't find something as good as an HD600 because of the budget you've got, but it's an understandable limitation that a lot of people have to deal with. Of course, not everyone has found the SHP9500 to be their cup of tea, but for the sake of getting a good headphone on a budget, I hope you aren't one of those.

 

[Tayyab Pirzada]

I personally really enjoy the bass out of HD600's. It's so boomy. I've heard details in my music I never heard before. I don't know if it's just me but the HD600's make even low bit rate tracks sound good.
 
So is the Hifiman HE-400i a noticeable improvement to the HD600s?

 

[Me x3]

  I personally really enjoy the bass out of HD600's. It's so boomy. I've heard details in my music I never heard before. I don't know if it's just me but the HD600's make even low bit rate tracks sound good.
 
So is the Hifiman HE-400i a noticeable improvement to the HD600s?

I don't think so, but your preferences are key once you've reach this level of sound quality.

 

[kman1211]

  I personally really enjoy the bass out of HD600's. It's so boomy. I've heard details in my music I never heard before. I don't know if it's just me but the HD600's make even low bit rate tracks sound good.
 
So is the Hifiman HE-400i a noticeable improvement to the HD600s?

Not really, had them both for a bit. I did like the HE-400i's speed, imaging, and better bass quality over the HD 600 but it didn't sound as right as the HD 600 to me. I liked the HD 600 more of the two personally. It's best to say they are just different and I can see some liking one over the other depending on preferences. 

 

[Phishin Phool]

  I personally really enjoy the bass out of HD600's. It's so boomy. I've heard details in my music I never heard before. I don't know if it's just me but the HD600's make even low bit rate tracks sound good.
 
So is the Hifiman HE-400i a noticeable improvement to the HD600s?

Glad you like it but I find the exact opposite . My HD600 has less bass than my other headphones (accurate but not neatly as forceful and the decay is markedly different but my other cans are v shaped as hell and have a lot of bass) and at times I EQ in a little extra bass. Also low bit-rate tracks to me are the bane and antithesis of the 600 as it is so analytical it exposes flaws in poor source material. none the less I do like my hd600 and it is as accurate as you can get at that price point IMHO.
I too am running a garage 1217 amp most of the time (project sunrise 2) and like it a lot with the PSVANE 12AU7 NOS tube I have in it now

 

[StanD]

  I don't have the Yamaha anymore, but it sounded quite well indeed. In particular with Beyerdynamic DT880 250 which has a less pronounced impedance bump in the mid bass region than the Sennheiser HD6x0. DT880 is also less warm so the added warmth can be a good thing, depending on the recording for sure.
 
I know 1dB is small, but what I've said is that a small 1dB bump ranging from 45Hz to 55Hz is much harder to hear, than a broader 1dB bump from 45Hz to 250Hz. Even then, it's small, but probably audible in the long run.
 
All my amplifiers have low output impedance, but perceived sound quality is much more complex than just a flat amplifier, so I would say buy the amp that make you enjoy your music the most.
 
Not sure what jdd means, but HD600 has quite high second order distortion in the bass, much higher than most amplifiers, and most people like HD600's bass.
 
HD800 has less second order distortion in the bass than HD800S, but most people prefer HD800S bass. (I do keep my HD800)
 
Pure objectivity for the sake of objectivity is a fine excercise. But enjoying the music is most of the times unrelated.
Mainly due to the vast disparity among recordings.

JDD means Just Detectable Distortion, which is the threshold of where we can perceive distortion. This number is much higher than what most modern Amps produce. Headphones, and especially loudspeakers produce more distortion than Amps, so for the most part, put your money into a good pair of headphones. As far as listening to or enjoying music, my opinion is in my signature, however, that doesn't mean that I'll simply repeat anecdotes about impedance curves and perceptible sound. I try to be accurate and informative, I'm an EE, designed plenty of electronic gear (pro audio), have a good ear and know the ropes.

 

[Drrizzt]

  do you think it's worth it upgrading my amp? I have a Project Starlight. I like it because the tubes add some bass and warmth to the HD600's.

 
The G109 is totally neutral and transparant, the upgrade is worth it to my ears, the gain in resolution allows some subtle details to be more apparant. Besides, The tightness of the bass is better, there's more air top end, the sound is slightly less in your face, it's a bit less fun sounding amp (one might call it "boring" i guess) but it's relaxed and detailled at the same time, with the polaris we sit at the second row, with the LP, at the 4th row. Discipline from germany in other words rulez them all
 
Lake people really did a awesome job with this ultra transparant amp packed with such power, looks like it's an o² with more testosterone. 

 

[Me x3]

  JDD means Just Detectable Distortion, which is the threshold of where we can perceive distortion. This number is much higher than what most modern Amps produce. Headphones, and especially loudspeakers produce more distortion than Amps, so for the most part, put your money into a good pair of headphones. As far as listening to or enjoying music, my opinion is in my signature, however, that doesn't mean that I'll simply repeat anecdotes about impedance curves and perceptible sound. I try to be accurate and informative, I'm an EE, designed plenty of electronic gear (pro audio), have a good ear and know the ropes.

"...I use equipment to listen to music."
 
I think that's fine, me too.
The important thing is what happens if you feel certain recording lacks bass for example. A non-linear reproduction system with raised bass will make that recordings sound more natural and music-like to your ears, then it would be a better piece of equipment to listen to that particular music, despite being non-linear.
 
Correct me if I'm wrong, you seem to say lower distortion is better, flatter response is better.
What I say is: recordings are all over the place, and same apply to personal preferences so lower distortion and flatter response is not always better for the sake of enjoying your music.

 

[Sonic Defender]

   
What I say is: recordings are all over the place, and same apply to personal preferences so lower distortion and flatter response is not always better for the sake of enjoying your music.

That is why you want to have as much of a known factor, to compensate for the unknowns. StanD is correct, flat and lower are ALWAYS better. Colouring sound to your preferences can be done in other ways, the headphone strikes me as an easy way to accomplish what we want with a sound signature.
 
Edit: I will revise what I said above a little. The distortion introduced by tubes can and often enough is perceived as pleasant sounding. I have a nice OTL amp and while we aren't talking night and day obvious differences, I can perceive the distortion from my tube amp and mostly I really like the effect. So I guess lower isn't always better, depending on what we are seeking and what we respond to.

 

[StanD]

  "...I use equipment to listen to music."
 
I think that's fine, me too.
The important thing is what happens if you feel certain recording lacks bass for example. A non-linear reproduction system with raised bass will make that recordings sound more natural and music-like to your ears, then it would be a better piece of equipment to listen to that particular music, despite being non-linear.
 
Correct me if I'm wrong, you seem to say lower distortion is better, flatter response is better.
What I say is: recordings are all over the place, and same apply to personal preferences so lower distortion and flatter response is not always better for the sake of enjoying your music.

I can change the FR to suite my tastes with EQ, I can have many EQ presets for different situations. An Amp that is not transparent/linear affects all recordings with no way out for all the good recordings. I doubt that adding more distortion to a bad recording is a good thing, at least not for me as IMO it messes up everything. I prefer that my headphones not have U or V shaped FR curves that some people call fun, which is not fun for me.
At the end of the day, do what pleases you.