Those adapters are what he meant by "a reamp".
A passive DI turned backwards works great as a reamp.
The main "sound quality" issue that could happen is distortion from lack of headroom. So you can totally solve that issue without any adapters just by turning down the volume on the line level device.
The one other tiny fine point is that the tone may be different between an active device (like a mixer) and a passive guitar/bass, going into the same amp. The passive one could be darker, depending on a variety of factors. The reamp devices can bring back some of that darkness.