My apologies for the late reply. We were trying to get shunt down for two weeks the day after your post.
I'm not sure you tried it the way I suggested. So..., please confirm: You combine the core from the Black & Red, and connected it with the shield from the White & Blue, on each end for the Positive. Then, you combine the core from the White & Blue, and connected it with the shield from the Black & Red, on each end for the Negative. You tried it with, and without the cable shield connected to the Negative of the amp (only).
For a better understanding, research DIY speaker cable receipts Jon Risch. We applied that approach to the NES-3003, and really liked the result.
If you have not tried the connection above, please do. Other things to consider. When using set screw connectors, you should always use the appropriate size crimp sleeve. This will make a better connection, and protect the wire from damage. With this setup, a spade connection will be better. If a spade absolutely can not be used, use a better banana. That 645 leaves a lot to be desired. It would not be hard to find a banana with a 50% increase in Copper content without that thick Nickel subplate.
BTW, the best, and fastest way to acquire tech from me is to email me directly. The address is on the Contact page at my website