Do not tap from your sound card's headphone jack when connecting to an external amp, otherwise you're amplifying everything generated from the mediocre built-in headphone amp in your card. Your sound card should have a line-out jack, use that, and let the external amp do its work. I don't know what sound level your card sends to the line-out jack. My M-Audio Audiophile card send 0dB to its line-out when I turn the volume control to maximum through software.
As regards to upgrading or tweaking the built-in headphone stage in your card, YMMV, I personally would spend my time and money on an external amp instead - if for nothing else it will enrich your experience in this hobby. I'm not discounting any tweaks you can do with your card; they can be beneficial like many have successfully done so.
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