WASAPI: Lost my bit-perfect output

[mcow]

I upgraded to Windows 7 a few months ago, and at the time I set up S/PDIF from the mobo into my HDCD DAC. This was working exactly as expected; if I sent a (lossless) HDCD bitstream to the DAC, then the HDCD indicator lit up and the benefits were clearly audible.

Now this doesn't happen. I get sound, but it doesn't sound like HDCD decoding is being applied, and the HDCD light doesn't come on.
I can't figure out what has changed. I've gone thru all the settings in the Windows sound and in the WASAPI plugin, and I just don't see anything changed, let alone anything wrong. I can (as expected) move the volume slider up and down, both in the player and in the Windows mixer, and there is no change in the output.
I've also checked that the DAC does still properly decode HDCD, via the optical input from a CD player. (The computer connection is via coax)

Has anyone encountered a similar problem, and if so, how did you fix it?

[Lowlander]

Have you tried playing the same file from Windows Media Player (or other media software capable of this) and see if the HDCD indicator lights up? If it doesn't it may point to a system issue (OS, hardware or DAC).

[mcow]

I found the problem: I had recently turned on Apply Replay Gain. Even with no gain value, the data must get scaled and the integer-math scaling doesn't come out exactly the same as input.