Answering the question if HD radio signals sound bad on non-HD mode, Doug T writes:
The answer is yes -- HD radio causes interference, and it is noticeable for AM. This is not a subjective question, it is a simple issue of electrical engineering. The FCC, in approving HD radio, apparently felt the trade-off was worth it.
At the time HD radio was approved, I was quite skeptical of both its business case and engineering case. I am very sorry that my skepticism has proved to be well-founded. HD radio has worsened the radio experience for the average listener. HD Radio has proved such a flop that the vast majority of non-car radios offering "HD" have been withdrawn. (Because of the way HD radio works, I think it is an especially awful engineering choice for car radios, but I am not sure what the actual market impact has been.)
What a farce.