would this mean the left RCA out plug on the DAC is in someway faulty?
Ya, that could be the problem.
Question is was the DAC RCA jack faulty to began with, or did the longer pin on AECO plug damage the center contact in the jack? Did you by chance first use your other RCA ICs on the DAC, and they worked ok then?
What make is the DAC? There is a big quality difference from panel mount RCA jacks and circuit board mounted jacks.
Jim