I think the upgrade was worth it- being able to listen to 24/96 music was worth it by itself. I did not do much A/Bing between the 2 but I love the uDac2 with my T-amp.- nice stereo separation and low noise floor.
Audio Technica M50s. I have both the M50s and the C551's- you will not be disappointed. Better soundstage and tighter bass compared to the Denon's (which are very good IEM's). Sound very good unamped, though even better amped. Look on ebay for very good deals.