With much deliberation, I pulled the plug and bought a NAD M51. With the budget I had set, this seemed like my best choice. Initially hooked it up to my tubed Anthem preamp and was underwhelmed. Left room and came back 5 or 6 songs later and the DAC really opened up. Finished the disc and powered down so I could hook NAD directly to my Rotel RB 1080 amp. Immediately preferred this setup. Listened to my system for two days running NAD into amp and enjoyed every second of it. Resolution was greatly improved over my CDP and DVD players. With NAD my soundstage was larger. Surprised me how much music was happening now outside the outer width of speakers, but more than anything the presentation sounded very natural to my ears. Incredible extension at both frequency extremes. Bass was most articulate and more satisfying than i've experienced thus far, even exceeding my analog setup.
So after this two days, I hooked DAC back up to Anthem and for the first time ever, realized how much coloration the tubes were giving me. Mids and high frequencies were sweeter with every recording (sometimes unnaturally so), but bass was tubby and bloated, lacking in what i'd enjoyed over the past two days. Soundstage also resumed it's smaller size.
Before getting NAD, I suspected my amp would stand out as being glaringly in need of an upgrade, but now I can tell my preamp will need to go if I ever plan on getting my analog up to snuff. Perhaps the coolest thing for me is just enjoying music i've listened to forever in such a satisfying way. I'm very pleased thus far and hope it gives me as good of service as my other gear.