I think your should consider replacing the pre/pro with a 2-chnnel preamp and a DAC. To do this on your budget, I would purchase the DAC 1st, and circumvent the digital processing in the pre/pro, using it as a pre only. One option, the Music Fidelity V-DAC II, often show up on AG or Ebay for ~$150, and has USB and Coax input. This leaves ~$350 for the preamp. A used Rotel or NAD can be found in this range.
Another option might be to replace both your Pre/pro and amplifier with an integrated amp. I have an NAD 526BEE which I like. New retail is $550. Sometimes these come up for sale, also the predecessor the 525BEE which is very similar and yet not to old a vintage, and would be less expensive.