[Discuss-gnuradio] adding a DC offset to the output of a LFTX

I'm trying to add a DC offset to the output of my LFTX driven from a USRP 1. Is this possible progromatically? I edited my GRC block diagram to include adding a constant value to my complex stream, before inputting into the USRP sink block. I have a scope output at the same point, which shows adding a constant value to the signal, but the output of the USRP doesn't match. What am I missing? I've tried adding both a constant real value, and a complex constant with equal parts I and Q - neither work.

Thanks.

-William