It all depends on the sound card. I would expect a "line" level of around 1Vrms full scale. 0dBu is 0.775Vrms, or 1mW into 600 ohms. CD players using the redbook specification output 2.0Vrms, I believe. Budget headphones (we're talking the $5 variety often used with computers) don't need this much voltage swing, but I imaging the headphone outputs capable of 1Vrms. Bascially, me thinks you can use a sound card or headphone output as a line level to plug directly into a power amplifier.
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