Mid-priced warm sounding DAC?

I've recently decided that a good route for me to take is upgrading my system with a good DAC. My current system is a bit bright sounding, so I'm looking for a good DAC costing up to $600 (used) that will provide a smooth, warm sound to match my currently bright system. I'd also like the DAC to be capable of upsampling, although if necessary I could add an upsampler to the digital chain at a later time. My preference would be to have it built-in, however.

Any suggestions?