Find a good R2R/multibit vintage DAC, like many mentioned around these parts or in purrin's HF DAC thread, or save up for the Yggy. Perhaps trickle down Yggy tech will be available sometime before you get the cash saved up for the Yggy. There might be a few good new DACs based on older DAC chips floating around too. A lot of other stuff is going to be more just a different flavor than an upgrade.
We need to really compile a list of good old multibit/r2r DACs sometime. Purrin's thread only touches on a few. I'd love to know about more.
I guess I should ask what you're really looking for over the Anedio D2. You mentioned lackluster treble, but I'm not sure in what way you mean that. Too dull? Tired of the Sabre timbre?
But then we'd also need a list of good usb/spdif converters.