Hi, I have a CBII and have tried it as a pre-amp in analog direct. It's just fine. However, I have not been able to compare it in an A/B test with other pre-amps. Also, I have not got a CBIII around to do an A/B with. I suggest talking to your Theta dealer for their input.
Just out of curiosity, why do you want to use the Theta as an analog pre-amp?
Just out of curiosity, why do you want to use the Theta as an analog pre-amp?