The Delta 66 has an average noise-floor of ~-103dB.
The UCX has an average noise-floor of ~-110dB.
 
If you playback audio thru the UCX (that was originally recorded on the Delta 66), it'll have the same noise-floor that was originally recorded.  IOW, playback thru the UCX won't increase the quality of the original recording.
The D/A of the UCX is roughly 6dB quieter... and that (as a single stage) won't translate to a very audible difference.
 
Where you'll hear a noticeable difference is when that 6dB lower noise-floor is multiplied across 24-48 simultaneous recorded channels of audio.  It's literally like pulling a veil of noise off the audio.

Another thing to mention:
 
If you buy world-class converters... and the rest of your gear isn't world-class (super low noise-floor, etc), then you won't reap the full benefit from those expensive converters.
Final audio quality is the sum of all pieces in the chain.