[quastar]

My O2 amp is connected to my Auzentech X-Fi Forte sound card.  Usually I don't like heavily eq'ing my more expensive headphones, but lately I've been experimenting with it, primarily with the bass boost that's available in the console.  I noticed that when I connect my headphone straight into my soundcard with the bass boosters and bass "volume" turned high the bass sounds full, powerful, and thunderous.  Simply put, I haven't had this much fun listening to my bass-heavy music in a while.  
 
When I connect them to my O2 with the bass on high, however, it gets distorted really easily.  I can't even turn the bass up half way until the sounds starts to distort.    
 
Normally I connect the 3.5mm to the line-out (green, front speakers jack), but when I insert it into the soundcard's headphone output I get the same volume.  So I'm wondering: by connecting a external amp to my soundcard, is the distorting happening because it's essentially double amping?  Is the sound card acting like a preamp?  Why is it that the bass becomes so bad on the external amp when I increase it through the software?  

 

[TMRaven]

Are you lowering the preamp on the eq?

 

[stv014]

Do you have the gain set to high on the O2 ?
Edit: even on the default 2.5x low gain, the 3 Vrms full scale output voltage of the card is too much for the gain stage of the O2. You need to either avoid using maximum volume on the PC, or modify the O2 for lower gain.
Edit 2: the important thing is that the gain stage of the O2 will clip at 7 Vrms or higher voltage, regardless of the volume setting of the amplifier. You should keep the product of the peak output voltage of the source and the gain 10 Volts at all times to avoid clipping (peak voltage = RMS voltage * 1.4142), and preferably stay 10-20% below the limit to be safe.

 

[quastar]

Are you lowering the preamp on the eq?

Sorry, I'm not very versed in audio in general.  What do you exactly mean by this?  
 
 
 
Quote:

Edit 2: the important thing is that the gain stage of the O2 will clip at 7 Vrms or higher voltage, regardless of the volume setting of the amplifier. You should keep the product of the peak output voltage of the source and the gain 10 Volts at all times to avoid clipping (peak voltage = RMS voltage * 1.4142), and preferably stay 10-20% below the limit to be safe.

 
I always leave my gain is on the factory default low, which is 2.5x.  I'm not quite sure I know how to optimize or control how much voltage is going through my soundcard or my O2.  How do I keep it at peak output voltage?  
 
Normally I leave my volume at max on my PC and control the volume through my O2.  With this setup, the volume dial on the O2 is usually between 9-11 o'clock.  When I plug my headphones straight into my Forte, my PC volume is turned down to around 30%.  
Now, with my PC volume at 30% and all the bass boosters on high, I was able to get the same thumping bass through the O2, but I had to crank the dial to near max around the 3-4 o'clock (on low gain).  I'm not exactly sure what this really means, but I guess if I want to use the bass boosters when listening on the O2, I'll have to lower the volume on my PC considerably.