Then they are kidding themselves
Actually they aren't. Loss of impact is the most common complaint aimed at passive controls (the exception being when the control is already in the power amplifier). Loss of impact is a coloration.
The problem is that in many circumstances the digital is making so much output that the volume control can't be run up to full volume, so the user is not able to find out how much impact is affected. And this doesn't happen universally. So like so many other things in audio- you just have to try it and see. But in this case, there's no point in an outboard passive control, since the OP has controls built into his amp. He only needs a switch box.