I've been buying 1200-yd. spools of Coats & Clark at the local Joann's when it's on sale or I have a coupon. I've not had any problems with it yet. Once I bought a whole box (or maybe 2 boxes) of Signature 500-yd. spools there at half-price. I really liked it, but ran out of the color I was using and needed more. When I went back to Joann's to get more, they no longer carried Signature. I searched for that particular brand and color name on-line and found it at Red Rock Threads. I bought a 3000-yd. cone from them. I was really pleased with both Signature Thread and Red Rock Threads (who carry many, many brands of thread). I can't remember what I paid for the 3000-yd. cone nor what I paid for shipping and handling, but I just checked out their current prices and see that Signature cones, 500-yd. are $4.75; 3000-yd. are $11.95 and 6000-yd. cones are $20.95. I THINK there are very few color choices on 6000-yd. cones. On top of that, add shipping and handling ($???).
I have a Janome 1600P DBX set up on a grace frame with a practice quilt on it. Unfortunately I have not had much time to play with it, but when I did practice a bit with it, I used Coats & Clark thread and had no problem with it. I hope it continues to work o.k. for me ... I have a half-dozen spools of it and I hate to waste anything.