There was a long thread a while ago (last spring maybe) with comparison clips of stock and modded UFX units. The thread was later deleted due largely to one pesky slut who was later banned.
The results were mixed. Many people could not hear a difference. For those who did, I recall that more preferred the stock vs. the modded as the former sounded slightly "deeper" and "more full." This was my personal assessment as well. However, other folks whose ears I trust found the "deeper" and "more full" aspects of the stock model were essentially "mud" and therefor preferred the mod.
Bottom line, the differences were subtle but real; but there was no clear winner sound wise.
Originally Posted by AwTAC
There is so, completely, in every way, nothing wrong with the stock unit... Ive never had the impetus to open my UFX
These are my thoughts as well. The stock unit sounds great, and differences among good converters are always very subtle at best. I would not pay for the mod unless subsequent tests show a clear benefit.