I would take it back. Just a quick thought for anyone taking a machine in for service - try to make some time so that you can sew on the machine before you leave the shop. Maybe even before you pay - I just thought of that one. I work at a shop part time (weekends) and I know that they have the girls do sew offs before they call the customer, but even then they may not be testing the same thing you are doing. It is so much easier to not take it home than it is to get it home, get around to using it and then taking it back.
This is what I did with my machine recently when I collected it. I made a time to sit down with the tech, I took the thread with me that i'd been having major issues with. we went over the settings and quilted on a small sample sandwich before I paid and took it home. They didn't seem to mind doing this.