[jprog]

I got a pair of RHA-SA950i on-ear headphones for Christmas and I've found them to be significantly better than the crappy in ear headphones that I was using before. Unfortunately, they make my head hurt after about 2 hours so I'm looking to buy my first pair of nice headphones.
 
Here are the things I'm looking for and information about how I listen
1. I'm looking to pay less than $150
2. I'd like to be able to wear the headphones for 6-8 without any discomfort.
3. I listen to classical, jazz, rock and roll and pop/rap
4. I need to be able to wear them in public places like the library.
5. I listen from my iPhone and computer and I get music almost exclusively through spotify (premium)
 
From the research I've done, it seems the KRK KNS 6400 or 8400 would be a good fit. The Sennheiser 558 also seem like a great pair for me too except for the fact they are open and I don't want them leaking sound in a public place.
 
The only concern I have with the 6400/8400 is that some reviews have said they are more "analytical" headphones so they may not be appropriate for people trying to enjoy music. Should I be concerned about this since I only want to enjoy the music? Are there other pairs of headphones that would meet the criteria I've set?

 

[KG Jag]

The 8400 is a very comfortable can for me and for most people.  However, headphone comfort is a most individual thing.  Only by putting them on your own head for that 6-8 (hours I assume) listening session will give you a definative answer.
 
The 8400 is a studio can.  I enjoy its sound signature, especially among closed cans in its price class.  However, some find it to be too clinical or dry for their taste.  Others find that it does not have enough bass.  Again, the only way for you to know for sure is to put them on your head and listen.

 

[Notus]

I am not sure what people mean by analytical in general, but i think they mean by analytical that they will show every flaw there is in music.
The KRK KNS 8400 have nice separation with instruments, fairly good soundstage for closed back headphone. I think they have great extension in both bass and treble. You could say they are slightly bass light but the bass is very tight and controlled. 
Its possible analytical headphone can take away from the music by revealing bad quality recording / mastering, but in my opinion with good quality recordings they sound very good. I do have some very poor recordings and the 8400 do reveal where the recording is lacking, but that does not stop me from enjoying music with them.
 
I got the KRK KNS 8400 for portable use and so far they have performed great. I have large ears and while the 8400 are small in size for over ear headphones they are very comfortable for me. The memory foam can take up to 5min - 15min to soften up with the heat of your body but once it does, they just melt away on your head. Out of box they do have fairly strong clamp but it does get better quite quickly. I would point out that i did not feel any pain using them out of the box even with the strong clamp and i tend to be sensitive to pressure on my head. 
Oh i would say they need a lot of time to burn in, can take up to 100h - 200h.
Out of the box the treble can be fatiguing and aggressive but at around 60h burn in that fatigue goes away.
 
I listen to a lot of orchestral music like Two steps from hell and some classical i think the KRK KNS 8400 work very well with both.
Also listen to a lot of Metal music. Hope this helps a bit.
 
Other than that i was considering the Akg K550 and the Brainwavz HM5 / NVX XPT100 / Fischer Audio fa-003 / Lindy HF-100 those are essentially the same headphone with different accessory's and slightly different color. 
I have heard good things about the Sound Magic HP100 but those were over my and your price range i think.
The Shure SRH 840 is in your price range with warm sound signature, they sound great but the comfort for me was very bad.

 

[Claritas]

The treble is crisp and too fatiguing for longer periods. Burn in didn't help enough for me, I guess. I would certainly use them in the studio though.

I agree that the Fischer clones are a good choice, offering a coherent and smooth sound.

I would recommend SRH840 with one of the headband mods. It's sort of the neutral version of fun.

Another possibility is a T50RP but if you end up not liking stock, you'll need to have the patience and talent to mod.