I have been having good luck with Signature thread but poly and cotton on my Janome 1600p. I went to Gammill sewing center that sell Janomes and talked to the person in the service department for the right kind of bobbins. I found out that the bobbins I had been using were the worng kind. I bought a dozen new bobbins and Organ HLX-5 size 16 needles and the machine has run perfect since. No thread breakage, and no tension problem. I amd going to post a picture of a 10-minute block quilt in a few days that I quilted on my setup. I had to do some ripping on it, but that was all my fault for forgetting to do a simple thing like lower the presser foot.
I have my machine presser foot tension set to 0 I do not cover the feed dogs, so the setting of O must take care of the feeddogs. I have the stitch length also set at 0 with the stitch reglular setting for the length of stitch I want. I use cone thread and have the threat guide pulled up as far as it will go. I had a cone on the back spindle for the bobbin thread, the the top thread goes on the fron spindle. I run my thread through the guide that you would use when winding a bobbin nover to the pretensioner before doing the rest of the threading.