After reading your original post but before reading your fourth, I was going to advise you to keep an eye out for tech drive issues. More commonly than they wear out, tech drive systems can be knocked out of adjustment. One symptom of this is that the vacuum will propel itself more readily in one direction than the other. Almost always, the cure is an adjustment of the attachment point between the handle pivot and the slide block underneath. It wouldn’t cost you anything to do it yourself, but it would be a pain.
I digress. The great thing is, you already picked one which drives well. 
Thanks for sharing!
