I cook that low at times... but rarely. (mostly for salmon, about 190, etc) In the past I would use 170-180 to give a little "extra smoke" to brisket, or to "smoke" some steaks ala the "reverse sear" method. However, I've found I don't really need to do that to brisket to get great smoke flavor on this grill, and the steaks do great for about 10-15 min at 200.
I've had the best luck at keeping it low if I "creep up" to set temp. So starting HHT at 20-25 will provide that "creep" to a low temp. However, I've heard several people make the argument to heat the grill to 300, then let it back down to the set temp. I don't know the best answer or which is faster, but I've had good luck with my "creep to temp" method. Another little trick is leaving the hood closed while starting the fire. Only downside to these tricks is sometimes you get a fire that's running too lean.
I've honestly had trouble trying to keep the grill below 200. But, my experience has been that the difference in 170 and 200 has been negligible in quality of the Q. Taking the flu cap off and making sure the grill is out of the wind (and sun) as much has possible has helped tremendously in keeping temps low. Honestly my best low temp cooks, have been when the ambient temps are <45 degrees.
(So... yes there was a little sarcasm in my "Quit cooking at 170" comment)
Take good notes, experiment, and call Cookshack if you can't get it resolved. They are always very willing to help.
Nordy