[MrMateoHead]

Quote:

What make and model is the motherboard?
Chances are adding any quality external amplifier would not increase the bass, but it should improve the quality of the bass.
As the HD598s are only 50-Ohm, a headphone output jack with an impedance of 5-Ohms or less is preferred
So you want the headphone's Ohm ratting to be at lease 8 times or more the Ohm rating of the headphone output jack
(Impedance is resistance, measured in Ohms)
Sound cards can have an headphone output impedance ranging from 10-Ohms to 100-Ohms.
Portable headphone amplifier can have an impedance as low as .5-Ohm
I'm of the belief that motherboard's on-board audio comes with a high impedance, but have no data to back that up.
You can always try something as cheap as the Fiio E6 portable headphone amplifier, to see if the audio quality is improved.

From the high-end cards I looked at, most used the same TI chip for the headphone output stage. It spec'd to 10 ohms out, making them less well suited for low impedance headphones. That said, other boards allow user selection of headphone impedance, meaning they have probably tried to optimize the chip for different load demands. Also, it really depends on power output as well - some of the cards are so powerful, they would still drive an efficient, high impedance phone well.
 
As some others have said to me, just because chipsets spec one way from the manufacturer, doesn't mean the computer builder is going to follow that rubric. I am just lucky that I have a low-impedance output as I didn't look for such things before. But I would agree that relatively high-impedance outputs are a better assumption - and anyone interested had better try to look it up!
 
Quote:

My question still persists if an amp will improve the signature of my headphones, perhaps increase the bass by a little bit, make it sound more clear, etc.

Sorry, but its the answer you don't want: IT DEPENDS! In fact, MANY of the USB/DACs have a high output impedance / current limitations which aren't optimal for low-impedance headphones. But they do compensate for this with gain switches, which make the amp "louder". Because you use HD650s, you should be fine with a Fiio.
 
My amp purchase DID have an impact on my overall sound, and yes a noticeceable part of that was a stronger-sounding bass response. But I would buy an amp because I needed more volume, NOT because I expected a significant change in sound quality. That I did get a noticeable change is probably because my phones are current-hungry little monsters.

 

[proton007]

What he said. ^^
 
I'll also add this if you're interested, just the jist of it. Headphones are a power dissipation device, and in order for them to perform well, you want all the power transferred to the headphone at all frequencies.
If you add more impedances along the way (amp output impedance, for instance), you're hindering full power transfer to the headphone. Impedance has resistive and reactive properties, which means, at some frequencies, you won't be transferring full power to the headphone, and it will sound weak in some areas like bass or treble.
 
So its good to have the lowest output impedance. Based on this, you may or may not find a change in sound.

 

[Catchytune]

Yes, probably. But don’t expect any miracles. My Macbook Pro for example has a noticable channel imbalance. Mostly at low volume though and not disturbing but still there. The bass intensity is higher when I connect my HD 558 directly to my CD player or hifi amp.

 

[stv014]

Quote:

From the high-end cards I looked at, most used the same TI chip for the headphone output stage. It spec'd to 10 ohms out, making them less well suited for low impedance headphones. That said, other boards allow user selection of headphone impedance, meaning they have probably tried to optimize the chip for different load demands. Also, it really depends on power output as well - some of the cards are so powerful, they would still drive an efficient, high impedance phone well.

 
The impedance selection on sound cards is usually just a gain switch, and does not affect the output impedance. In the particular case of the Xonar cards, it does not even change the analog gain, it is just a digital attenuation so that the sound output is not too loud with sensitive low impedance headphones.

 

[stv014]

Quote:

The bass intensity is higher when I connect my HD 558 directly to my CD player or hifi amp.

 
That may actually be caused by those devices having a higher output impedance than the MacBook Pro, to which the HD5xx are quite sensitive.

 

[Frosty3258]

if your source has an audible inherent hiss get a dac first, if it is clean sounds like you want an amp more so get that, i personally like tubes with the hd598