i used to use an ase z-man tubed buffer stage w/my nad 515 (and the panasonic portable slp1a before that). this was the *only* way i could tolerate the digital nasties, when it was run thru an electrocompaniet ec-1a preamp, & then, a linn kairn preamp. but, since i've gone to a tubed preamp (1st rogue 99, then cary slp98, finally settled on a melos music director), i've found the nad 515 sounds fine run straight-in. in fact, it gives up *nothing* to the $1800 alchemist cd-player, & it gives up nothing meaningful to the $3k resolution audio cd-50.
to be honest, i was hoping these spendy cd-players *wood* be a big improvement (or even a small improvement!), as i really find digital sound doesn't compare favorably w/winyl. however, that was not the case, at least in my system. w/a tubed preamp, tho, at least cd is enjoyable for me, now...
ymmv, doug s.