bfw:
I have pondered the same thing - I recently bought the Pre XPD with the QX upgrade and wondered if I plugged my CD8SE into the preamp digitally would it sound better than via the analogue inputs?
I was at a Cyrus day in my local dealer last year and I asked him about the DAC's in the amps/pre amps v's the built in DAC in the CD player and he said that although the DAC in the 8XPD/ Pre XPD were the same as the 8DCSE they preferred the sound using the CD players own DAC.
Cyrus claim the QX upgrade is in between the DAC in the CD8SE and the DACXP.
Worth an experiment - If I were you and rather than spend a lot on a coax cable get an optical from Lindy as below
http://www.lindy.co.uk/1m-spdif-digital-optical-cable-toslink-premium-gold/35241.html to see if you like the sound and then invest in a good coax digital.
As an aside I bought a Toslink to mini toslink (Lindy) and played HiRes FLAC 24 bit 48 khz from my Mac laptop through the PreXPD and it was stunning!
BFW,
Have you compared the analog output of the CD8SE to the Qx DAC directly in your system at all? I upgraded my 8XPd to the Qx DAC and noticed a considerable difference, on both CD and PC files.
Incidentally, the 8XPdQx is off to Cyrus to become a PreXPdQx. Will be looking for a new power amp (amps?) soon...