i understand that there are folks who prefer their digital run straight-in. perhaps, as in your case, the tube amp is a key that helps smooth-out what i find to be etched, fatiguing digital sound, when run thru solid-state equipment. i also find it telling that ewe say "it sounds as good, if not better than" when using a pre. at least, in your case, the pre doesn't seem to be degrading the cd-sound. for those of us who use other sources, this is good to know.
prior to getting my current tubed preamp, i had a linn kairn feeding signal to my solid-state electrocompaniet amps. the only way i could tolerate digital, even when run thru the linn, was w/an ase z-man tubed buffer-stage w/nos mullard tube. i don't need that buffer-stage anymore, & as i said, even my cheap cd-changer now sounds as good as i'm gonna get for any reasonable amount of money.
regards, doug