hi windbrit, I'm don't use an equalizer as my alpine's source parametric eq is good enough for me, so I'm not very skilled on this devices but I would not expect to modify the preamplified signal, as usual mount is between source preamplified output and amplifier RCA input.

A nice source should give 2V, or even 4V preamp output, I would bet this is not the case for the audio card output by far, I guess there are two options, use a "source" instead of an EQ which can use audio card signal as AUX input and then feed the ampplifier from RCAs form the "sourc" preamps, or tune the ganance (I don't know if this is the right word in english) in your amplifier, some builders call it input sensibility to inputs of less voltage, this is faster and most probably cheaper, although as you know as you increase ganance in amp. the output quality slightly drops.

Hope it is usefull

PS: how do you control volume???