Most people won't notice the difference if there are only two HD channels HD-1 and HD-2. I can, because I have better equipment. By they time they add an HD-3, all channels are suffering to point that analog FM sounds better. Typically - stations devote the same bitrate to all HD channels, but I've heard very low bitrate on HD-3's dedicated to NOAA weather radio and the like. You are right, they can allocate the bit rate any way they like. So assuming there are two channels, HD-1 and HD-2, the HD-2 will sound as good or better than an AM stereo station broadcasting in C-Quam and heard on a decent AM tuner in a low interference area.