Don't have them side by side to compare, but I found the OMZ (I have 2 of them, a silver-faced D2.0A 4.0 and black face D2.5A 4.1 w/headphone amp) to be VERY transparent and have a big soundstage.
In MY systems, the bass is bettered by other DAC's. I'm using them both with decent power conditioners, but others have reported improvements with their conditioners.
I have heard an earlier revision of the DAC1, but I don't think it's reasonable to comapre them, unless you're getting used (in which case, the early DAC1 was a little too edgy for me).
Haven't heard the X-DAC v3 at all.....
One more consideration: CIA VDA-2 w/ or w/o VAC-1 power supply. Supposedly a great DAC; but I've only read others' posts about it.