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[179831]

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[144004]

I don't know the science of output impedance vs headphone impedance, but I really recommend you trying things out without an amp first.
My 250 ohm Beyers sounded fine out of an M-Audio interface; the interface just couldn't go as loud as an amp.

 

[PleasantSounds]

Quote:

Amping (or the lack thereof) was a big reason that I decided on the Senn 598s.  Go much higher on the price point ladder, and the headphones start asking me to shell out for a pricy amp.  For this reason, I see the 598s as the "sweet spot".  Still, it would be hard to find an audiophile that believes my headphones wouldn't prefer an amp.  So I'd like to buy a cheap one that can sit on my desk.
 
But my desktop has another shiny metal box incoming: I intend to purchase an audio interface (I'm looking at the Presonus Audiobox and the Focusrite Scarlett 2i4, among others).  These things are just tools for hooking up pianos, mics, and guitars to computers.  They have the capabilities of a lot of different gadgets: DACs, ADCs, sound cards, midi interfaces, etc., and throw in some gain knobs and line in/outs.  What's relevant here is that they provide a headphone jack.  I expect this to be a decent connection; an audio interface will bypass the cheap sound card in my laptop, and run everything through a passable DAC.  
 
What I'm concerned about is amping.  One of the audio interfaces I am interested in lists a meagre output impedance of < 12 Ohms (though it also says "power output to 32 Ohms = 24mW").  This doesn't match the 50 ohm sticker on the Sennheiser 598s.  So I have a couple of choices.  I can either get an audio interface and a cheap amp.  Or, I can scale up my audio interface to one that supports 50 ohm headphones.  
 
The questions that I need to ask in order to make this choice are as follows:

1. What are some of the more inexpensive options for amping the HD 598s?  I'm thinking definitely under $100, is that unrealistic?  Suggest a budget in proportion to the quality and needs of my budget-midrange headphones.
2. Would a 50 ohm audio interface do a good job of replacing a cheap desktop amp?  I know there are more factors to consider than output impedance when it comes to amplification, but that's the only measurement I have on these things in that way.  Strangely, there don't seem to be too many places where audiophiles and music creators mix too thoroughly, so I can't find a good description of the capacity of an audio interface as a headphone amp.

 
I think you are making one assumption which is wrong: that the output impedance of the amp should match the impedance of your headphones? In fact it should be at least 8 times lower than the hp impedance. In case of 598s that means less than 6 ohm would be ideal.
 
Before you jump in with an open wallet, I'd recommend getting the audio interface that suits your purposes best, without too much consideration for the headphone output parameters. When you get it give it a go as is - chances are it will work just fine. Start looking for an amp only if you feel the headphones should do better.

 

[179831]

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[PleasantSounds]

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Thank you both for your insight.
 
I obviously don't have a very good grasp on amps.  So the headphone impedance should be eight times the impedance of the amp, eh?  Is that the general rule?  How much wiggle room is there?  Is there perhaps a guide somewhere on this?  
 

 
It's just a rule of thumb to be applied when estimating how well the two devices may work together if you are not able to test them. Generally if you follow it you should be OK. There are some other factors that impact this matching, however I haven't seen them spelled out. Probably the headphones efficiency or sensitivity also plays role: the more sensitive the hp the less you need to rely on the 1:8 impedance rule - but that's just my theory.
 
There are hp/amp combinations that according to this rule should not work well but they do - that's why "try before you buy" is the recommended approach. In your case, even if it turns out that you need an amp, it won't have to be a massive powerhouse and you should be able to get a decent one without breaking the bank.
 
To me it looks like the audio interface selection should be based on different criteria. If you add the hp output parameters to your requirements you may find out that it narrows down the pool significantly, and you may not like the choices left in it - either because of price or combination of features not really fit for your purpose. Amping your headphones can be addressed separately if required, so I would treat it as "nice to have" as far as the audio interface goes.