I know you said that your dealer has been helpful, and has been out to the house to check etc - have you tried contacting Pfaff directly? Their educators are pretty good.
I recently had a situation with my machine and the local dealer was "so helpful", came to the house a few times etc but the problems went unsolved. As the machine was brand new, I got frustrated and contacted the company directly - turns out they wanted to see the machine in their shop - long story short - they had been asking the dealer to have it shipped back but the dealer wouldn't tell me that.
It might be worth a try, especially since you have had tension problems right from the start.
My machine was the same brand too, but never had tension problems, it was a software glitch combined with a motor box problem - all the problems are solved now, and so quickly once I got the manufacturer rep involved.
PM me if you want further help in following this path,,,,,
Good luck,