A few of us have used receivers with our headphones, mine was probably cheapest of all but it did a great job even compared next to my other dedicated headphone SS amps. Mad Lust had either that model you have now or a slightly lower-end model (Marantz just doesn't make really low-end stuff), I think it was even during the time he had the Annies, and he wrote comments such as "more refined," "effortless power," and I think "smooth." I think it would've been a fairly endgame component for him if his second-hand model hadn't crapped out and died on him. So yeah, it is a good pairing for the Annies. It also seems like you understand a lot of the benefits of the receiver over a stand-alone device... If I had more money I'd myself like to track down a model like yours in good shape.
I believe @Evshrug
has the Omni currently, if I'm not mistaken. If you're going optical out to another DAC then all you care about is getting the Dolby Headphone from the U3, and if it does that portion just as well as the Omni then there's no reason to spend the extra. But if you're using the DAC in the Omni, it's better than the one in the U3, yes.
Jury's still out on a comparison to the SB Z, and if the volume knob detracts from audio quality like the ACM module does... I think the volume knob just changes the setting of Windows volume anyway.