Hi,
You could try routing the analogue out on your creamware card directly back into the analogue input (without the compresor) to see if the signal degrades 'significantly'.
(By the way, you can also test your audio card's loopback quality/frequency response with
Rightmark audio analyzer )
If the direct loop back shows no problems then the problem is unlikely to be the audio interface or converters (though they will of course affect the signal, it should not be that 'drastic').
Another possiblility is that the bypass on the compressor is not a true bypass and and is affecting the signal when 'bypassed', but you didn't mention which compressor is being used (some poorer implementations of guitar pedals suffer a similar problem).
One further possible cause is the latency of the sound as it is converted to and from an analogue signal. This could mean that the returned signal is 'offset' by some samples which could cause a perceived drop in volume particularly if the tracks being summed are left/right of the same source. You can test if there is an audio offset issue by using a loopback again and simply playing a track on one channel out through one output and recording it back on another track in you sequencer, then look at where the recorded samples are in relation to the original.
If this is the problem, then note that some DAW software such as Cubase SX3 has a built in latency compensation for such hardware offsets, so check your software manual to see if it can do that.
Hope that helps.