I've done shopping around for the last few days and I've settled on Sennheiser HD650's (although I might change last minute to 600's). I'm fairly new to this whole thing and I was wondering, with the Schiit Magni 2 Uber amp that i'm told these headphones need, should I also spring for a Modi 2? Is there some huge difference between having and not having a DAC? Would it not be worth to spend all that money on $350 headphones and not have a DAC? For that matter, what difference is there between the HD600's and cheaper headphones? Is there an increase of quality as price goes up?
 
Thanks for the help. 

The 650's do need an amp and they perform better with higher quality amps. Although quality increases with price tags, it's not an even ratio. Diminishing returns set in quickly, so spending twice as much will not get you something twice as good.
 
To answer your main question, a DAC is always going to be in your signal path, if you are listening to digital music. You can't get away from one, unless you go analog (AKA a turntable). Your question is really: "is a DAC upgrade necessary?"
 
Since the most common audio source hardware (computers and phones) tend to use the cheapest components possible, upgrading to an outboard DAC is usually a good upgrade. How worthwhile it is will be up to you, your ears and your equipment.
 
Assuming that you are going to be using a line-out jack to connect to your amp, you can certainly save the DAC upgrade for later. If you're new to the headphone hobby, I'd suggest upgrading slowly, piece by piece. You'll probably end up with a better sense of the subtleties between different pieces of gear and music. You'll basically train your ears to listen better.
 
As for the difference between the 600 and 650, it's best to audition both and let your ears decide. Barring that, you'll have to read about what other people say about them and how they reproduce different kinds of music and make an educated guess.

Should I be spending more money on the amp? Is that Magni 2 sufficient? I mean I know it's functional, but when I was looking at the buying guides most of the amps were more around the $1000 + range. 

gibenw0lf said:

  Should I be spending more money on the amp? Is that Magni 2 sufficient? I mean I know it's functional, but when I was looking at the buying guides most of the amps were more around the $1000 + range. 

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for hard to drive headphones you will definately need an amp.
 
A dac however, it depends, most motherboards have a great dac and most people wouldnt be able to differentiate between a 1000$ dac and onboard computer sound (source: http://www.tomshardware.com/reviews/high-end-pc-audio,3733.html)

So I need to step up from the Magni 2?

gibenw0lf said:

  So I need to step up from the Magni 2?

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no i think the magni would suffice for sure, could also check out the Objective 2, basically the same but different form factor, get whichever is cheapest

Do you have an opinion on whether I should go for something better than a Magni to drive the 650's?

gibenw0lf said:

  Do you have an opinion on whether I should go for something better than a Magni to drive the 650's?

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I think the magni would be more than sufficient it will drive them.

There's no hard and fast rules about how much you should spend on this versus that. Later on, you can worry more about synergies and how well your particular gear matches. For now, an amp with enough power to run your chosen headphone to a loud enough volume should be sufficient.
 
I've always found that the headphone is the most important part, the link in the audio chain that makes the biggest difference and can vary the most from headphone to headphone. The amp comes next, then the DAC.
 
You should be able to tell a difference between plugging your headphones directly into a computer's headphone out, for example, and listening to it from a decent amp. The same should be true when you add a dedicated DAC to the mix.
 
For now, you'd do well to pay attention to "best bang for your buck" gear (of which Schiit gear is a good representation). So, yeah, start with the Magni 2 and grab whatever headphones grabs your attention.

fiio e09k would be a good option as amp too, might be cheapest

Oh and two more questions: When people say that tube amps change the sound, what do they mean? And should I consider a integrated amp (which I've been told is a DAC\amp combo)?

I always recommend testing with and without various DACs in your system.
 
Some DACs have sounded the same to me as my laptop's onboard DAC, and in at least one case, I preferred the onboard DAC over an external DAC!

gibenw0lf said:

  Oh and two more questions: When people say that tube amps change the sound, what do they mean? And should I consider a integrated amp (which I've been told is a DAC\amp combo)?

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They color the sound (add distortion) to make it sound, in most cases, warmer.
 
If i were you i'd get a solid state amp and choose the headset to change the sound

When I was researching amps I considered the e09k but settled on the Micca Origen.  I don't regret it.  It is small, powerful, sounds good, and, most importantly, has powered everything I have plugged into it.

Amazon says they're fresh out forever of the e09k's