I own a Hickok 600 and so far I have not found any tube I cannot test on it including 6DJ8/6922. The roll chart that came with it does not have these listed, but you can get updated roll chart data off the web. These are really excellent, just make sure you get one that is in good condition and you are sure is well calibrated. Here are some URLs for you to look at:
http://bama.sbc.edu/hickok.htm
http://bama.sbc.edu/hickok.htm
http://bama.sbc.edu/hickok.htm
http://bama.sbc.edu/hickok.htm