I have a Janome 6600 and have also found it to be picky about thread...and you're right, for a while I thought it only liked Guterman (or poly threads, like Sulky Polylite).
However, I discovered that it's very happy when I use Superior's Bottom Line in the bobbin (sometimes I also use it in the top).
Also, many problems were solved when I had it services (the guy did lots of things I could not due myself). At this point, it's fairly flexible about thread.