My pair works perfectly fine and there hasn't been any widespread reports of build quality issues. Two anecdotal incidences don't constitute a widespread problem, and I searched the X1 thread and saw no other mentions of bass rattle.
The driver matching is fine and comparable to the other headphones I've tried. Pricier headphones still fluctuate on high frequencies, and it's not something you'll ever hear outside of a frequency sweep.
did you try bass shaker test on that page using an amp and high volume ??