I purchased my UX-3SE universal cdp new over 5 years ago and consider it a fabulously musical cdp. I've not tried an external DAC nor have I had the desire.
On the other hand, about 3 years ago I purchased an Esoteric external G-25u reclocking unit. Over a period of about 4 months I tried various configs and I was convinced the unit did little to even nothing so I returned it for full refund.
Recently I attended a shoot out of 4 DACs and the setup was at a mastering engineer's home and system so he was able to have all units powered on, ready to process, and through his board he was able to instantly switch to any one of the dacs which varied in price from I think $800 to about $8000. I suspect everybody there could have benefited from the use of a stethescope to better distinguish any differences between the units.
One unit seemed a tad better here, another a tad better there but it was not unanymous and there always seemed to be a sonic compromise between them. To me the 4 DACs sounded far more identical than they did different.
My take is that your X-03SE has the potential to be extremely musical and more than satisfying as-is. You just might find a DAC that is a tad more refined but when you consider the additional cost of the DAC, the additional rack space required, the additional cable, the potential signal compromise from one more set of cable connections, the cost to properly treat the new DAC's noisy AC and the air-borne and internally-generated vibrations at and in the additional chassis I can't imagine too many DACs being worth such effort not to mention any ROI.
If you're bored or dis-satisfied with current system performance, I'd suggest auditioning some dedicated passive line coditioners (that actually do what they are supposed to do), or purchase some cryo-treated fuses, bulk power cable, wall plugs, and IEC connectors, etc. I'm confident you'd get far more sonic benefit just by cleaning up the noisy AC than from perhaps any external dac. And at perhaps less than half the price.
-IMO