Transporter modified by Dan Wright (so it becomes a tub DAC).
You get very good DAC as well as an easy introduction to computer audio. I have not heard it, but Srajan Ebaen just gave it a blue moon award on sixmoons.
http://www.sixmoons.com/audioreviews/modwright3/transporter.html
Quote: "this ex owner of Zanden and reviewer of AMR, Ancient Audio, APL Hifi, Audio Aero, Esoteric & Co. would be lying if he didn't call today's silver machine with its weirdly coexisting twin WiFi antennae and three valves their full and unequivocal equal"
Indications are that computer based audio is going to be where the higher resolution - up to 24/192 - will be released. The Modwright will be a wonderful Tube DAC for your Denon in the meantime. At $3600 you could get a computer (Macs are simple and silent, but any will do and it doesn't have to be in the listening room) to run the slimserver software and still stay way below your budget.
I am happy with my switch to computer audio (iMac + Benchmark DAC1) even though I still have a universal player for SACD and the occasional new CD that I want to hear before ripping.