Dave the PCM63 is very similar to the PCM1704k I use, for a good zero feedback I/V stage to implement into your Theta ferret through this lengthy thread (attached) I started on the AD844 used in zero feeback mode for I/V duties.
For your R2R 20bit PCM63 dac's you have in it, they are spec'd at 2mA @ 1.5kohm and would need a three stack of AD844, and use it's own output buffer that's built in also three stack, then you will have magic with RedBook replay.
http://www.diyaudio.com/forums/digital-source/227677-using-ad844-i-v.html
Cheers George