Having sat down to listen to music for the first time in ages I noticed that I am no longer getting bit perfect output from my RME card... .... I know all is well as the little light comes on on my DAC.
And you think the red light means "bit perfect"? I don't think is possible for the DAC to know if the S/PDIF stream contains bit-perfect data or not.
More recent versions of Foobar2000 have built-in HDCD detection and will convert to 24-bit. I'm not sure if this is standard or by default, but if it was happening it would certainly explain the light being off.