Brad, to the limited extent that speaker cable performance can be predicted based on technical parameters, with resistance and inductance usually being the most important ones, biwire Signal Cable Silver Resolution is an excellent choice for a 21 foot run, given the impedance characteristics of your speakers. I'm less certain about the Clear Day Double Shotgun cables, even at 11 or 12 feet.
In saying this I'm assuming that the goal is for the cables to have as little effect on the signal as possible, as opposed to acting as tone controls to some degree.
I've been puzzled by the technical descriptions provided at Paul's website. It indicates that the conductors are 24 gauge solid core silver, with the double shotgun version using 4 conductors for + and 4 conductors for -. In silver that would equate to a little heavier than 18 gauge for each conductor (closer to 18 than 17). Yet a separate indication is provided, apparently referring to a single pair of conductors, which states "resistance = 0.00325 Ohms/foot run." That corresponds to a much heavier gauge than even that double shotgun equivalent, and one which I would not expect to be achievable in pure silver at anything close to the prices Paul offers.
Al, however I'm guessing that for the long length you would suggest running two pairs for the biwire as opposed to one pair and jumpers (would this effectively lower the gauge?).
What would lower the gauge more effectively would be to run two pairs in parallel, in other words, with the jumpers in place. Although since biwiring is alleged to often provide other benefits, it might be worth trying it both ways, with and without the jumpers, using a variety of recordings.
Regarding the JPS cables, I don't see enough technical information at their site to comment. Regarding compatibility if and when you change to a tube amp, perhaps all that can be said is that the more closely the cable approaches behaving in a neutral/accurate manner in combination with your present amp and speakers, the more closely it will do so with the tube amp.
The bottom line, IMO: While you may or may not find the Clear Day cables to be subjectively preferable to the present Signal Cables, if neutral/accurate behavior of a long length is the goal I would have a higher degree of confidence in Signal Cable. Given, as I said earlier, that the predictability of all of this is of course limited.
IMO. Best regards,