My Harmon Kardon receiver decodes HDCD signals, and lights a nice HDCD logo when I put one in. I only have one HDCD - Roy Rodgers "Slideways". I can't hear any difference.

That's the point. The extra 4 bits don't increase audio fidelity in any way. They actually just contain subliminal messages which tell you to purchase more HDCD's and other Microsoft-branded products.