ONKYOTXNR609, USB, FLAC and CD Quality

[LaZ Baz]

Hi all,
This was posted elswhere but I have had no responses. I hope this will have better result.
1. I am ripping CDs to FLAC and saving on 1TB external HD.
2. I bought OnkyoTXNR609 AV Receiver because it has USB input and plays FLAC.
3. On test, the FLAC did not live up to the SQ of the original CD played on Cambridge Audio 650BD through the same receiver.
4. What could be the cause of the difference in SQ?
5. What set up change will be required so the FLAC will sound the same as the CD? (FLAC lossless should not lose any quality).
Any suggestions ?
Regards