Does a DAC completely bypass the computer soundcard

Yes

If you take USB output from the laptop, you will be bypassing the computer’s sound card completely. If you are using a reasonable audio player program on the laptop, what comes out the USB port (and into your DAC) should be bit-perfect.

You mentioned iTunes; I have not used it in many years, but it has had a bad reputation, so I'm not sure or not whether it fits what I mean by "reasonable audio player program."

Thanks. So Today I bought a Cambridge Audio DAC Magic 100, and yes! finished the ugly compression that was ruining my laptop music. It is now dynamic, crystal clear, with a beautiful stereo image. Very pleased with the result. However, I can really notice now the (sometimes huge) difference in the recording levels across all the files in my iTunes. The compressor was hiding more or less those differences but it’s much better without it. I recorded a 90mn tape on my Technics R2R RS1500US and the result is fantastic.