I am running Debian Sid. I've tried REW both in WINE and with the Linux installer but the issue is the same. I know the problem is not with Linux because I can play music at the same time REW is attemping to check levels but failing. If I turn up REW to max in the software mixer I hear a bit of noise during the level check, but it's nowhere near as loud as it should be. I had REW working previously on this machine, so I know it can work. Just to check if what I want is possible with REW: Is it possible to find frequency/phase response, then do a single tone THD measurement, which is then normalized to the freq/phase response, and then export the result as a .wav file? Is it also possible to normalize it to the loopback audio signal rather than the REW output?