In my DM machine I used YLI and in my HQ I use King Tut most of the time, I am using up the small spools of YLI on baby quilts. I have a couple of YLI cones; these are all variagated, which I use 99% of the time. :D

I just used Mettler silk finish variegated for 2 baby quilts, and was very happy with the results.

I love variegated thread and use it all the time. My preference is Poly as it is stronger. Our quilts have many colors so why not the thread! YLI makes some great colors and they make one for Linda Taylor, Thread by Design which is my favorite.
I use the Towa Bobbin Tension Gauge, which is worth every penny I paid for it!

Poly is just so much stronger. I don't really notice much of a sheen. But if you want sheen, it's out there. I had a gal at a class who had an issue with the variegated on top not matching with the bobbin. That blew me away as it is not possible to do! But you can do top and bottom or just bottom or just top!

I also love to use the veriegated threads. I have used all of the above, but I prefer the Robinson-Anton cotton or poly.

I just finished a customer's quilt using King Tut variegated on the top and Coats and Clark variegated on the bottom. It's true, you can mix and match.