When you split the bi-wire cables, you had half the gauge going to each set of posts. Now, you have doubled the gauge to the speaker via speaker cable. That is a huge difference in performance and is imo the greatest reason for the improvement.
I always do that when I use jumpers, that is, use a pair of speaker cables to the preferred set of posts for the input. You can reverse it and remove one set of cables from the posts, and you will have a diminishment of the performance globally.
And now, the coup de grace, are you ready? There is a superior option better than what you just did. It would involve four pair of speaker cables, yes, double bi-wired. It is another step above doubling a set of cables with jumpers. Most would not do this because of cost/availability of cables. However, if I am not mistaken, you have plenty of cables. ;)
You are correct; the quality of the jumpers is very important. :)