Didn't do Combank for Step 2, but did Comquest. Many of the Comquest answer explanations have a reference with them. Like was alluded to earlier, if you comment on a question, you will get a response from a staff member, most of the time with a reference link.
When I was taking the exam, it felt like I was taking just another Comquest 50-question set. I felt like the Comquest was representative of both the question style and the difficulty of the actual Comlex. My score on the real deal correlated pretty darn well with how I did on Comquest. Again, I did not do Combank, but after taking the exam, I feel confident that I would not have gained anything by doing Combank. Didn't trust Combank for step 2 because I felt like it was a big waste of time and money, except for the OMM questions, for step 1. Worked out for me.
Full disclosure: I also did most of UWorld as well...highly recommended that you do it too.