Yesterday and today I've been testing the UD-501 (still stock unit) against a Musical Fidelity A5.5 Cd with all the stock electrolytics just replaced with Nichicon (audio) caps and with the stock tubes replaced as well with (nowadays quite expensive) NOS vintage tubes. You can believe me, it's a great sounding cd player.
I connected the CD coaxial digital output to the DAC with a coaxial cable so that the source was the same, a cd. Both coaxial cables from CD and DAC to the amplifier same model and brand. I didn't check the outputs with a multimeter but dind't need to play with the volume, to my ears same level from both. So I could just switch with the remote from cd to dac and viceversa at any second.
I played some cd's that I know very well and are all great recordings. Different genres. For instance:
Smog - A River...
Pink Floyd - DSOTM (20th anniversary ed.)
Lambchop - Ohio
Screaming Trees - Sweet Oblivion
Electric Wizard - Witchcult Today
I will keep it short cause I don't have the technical vocabulary and don't want to make a fool of myself. Only in the bass area I like the cd better than the dac. Bass guitar low notes and drum bass for instance, go lower and you can hear and feel physically its impact much clearly from the cd player. Other than that, the differences are really subtle. In some songs the cd sounds a bit more detailed or with a slightly wider soundstage, but in many i was switching A/B and couldn't hear any difference at all.
To sum up, I think at the prices one can get hold of this DAC second hand (i paid 300€ for mine), it's an absolute bargain. Very fine piece of equipment.
I'd love to replace the power supply caps with big Nichicons KG's but there's no room, so the idea is trying Nichicon KA's instead. My primary target is to get that little bit more of bass.