They are hard to find quickly, but usually a Stax SR-Lambda (normal bias with six pins) and an SRD7/SB adapter that hooks into a speaker amplifier to drive the phones can be found for under $300. Then use your Qinpu or buy a T-amp such as Trends TA-10 or the Dayton/sonic impact T-amp to power the SRD7 for between $50-200 and you have an amazing starter unit for under $500. The pro bias adapters go for between $250 to $300 usually and can blow your budget there.
You could also buy a used ESP950 and use the E90 driver unit that comes with it for $500, but I don't consider it to be as high of a value with a stax adapter and Stax energizer.