Sorry but this makes no sense. Any noise that's below 20kHz will remain below 20kHz. Recording to 5.6 MHz DSD then converting the file to 384/24-bit PCM will not move noise from one frequency to another (if it did then the music would move too!). The noise above 50kHz you're removing with your next step is shaped noise added by the DSD format. Any background noise from the original vinyl will remain at it's original frequency and level (otherwise you wouldn't have to sum the bass below 30Hz to remove rumble!).