Hi,
I also have a Janome 6500 and love it but I agree about the thread, Coats and Clark just isn't the best quilting thread. Use a good one in bobbin and on top. My favorite bobbin thread is Superior's Bottom Line....very thin, durable and a filled bobbin seems to last forever. It can also be used on the top.
For the top thread I like Superior's King Tut, but my favorite cuz the price is wonderful and the thread is awesome... Connecting Threads Essential. There are 1200yds to a spool and you can buy it in 'sets' of 10-15 spools and they are less than $2 each! Egyptian Long Staple Cotton, does not knot up, no fuzzing lint. Check it out online. Not available in stores. http://www.connectingthreads.com/essential+quilting+thread+sets_EL600901.html
Hope this is helpful.
let us know what you end up using. Some machines don't like certain threads. Since we both have a Janome, you might consider my recommendations.
sue