You asked for used, but if not absolutely opposed to new, the Maverick Audio tube DAC /Preamp /headphone amp might be just the ticket. It's $200 and you could roll to a better tube later down the road ... like a GE 5-star 5670. With your set-up, you can go digital-in with the Oppo, put a basic Pro-ject pre between the Maverick/Rega and you're in business without dropping a whole lot a coin in the process. In addition to headphone amp, the Maverick D1 even has line-in for iPod. I have this great little "Swiss Army knife" component anchoring my office system.
