Cheap soundcard for measurement

More a matter of the smarts of the tester than the precision of their tools. For many tests, the measurement system hardly matters at all if you are working iteratively.

For example, you find your treble a bit bright. Do a trace. Use a bit of EQ. Do a trace again so you can know what effect the EQ had (and so you can return to an earlier setting once you bracket your preferred setting).

Even the 50-cent mic built into your laptop may be all you need. Certainly is a good place to start.

Mic versus mic: too bad for casual builders - diyAudio

B.