It might not be an issue, but some of those "ribbon" or flat cables can have really high capacitance (like Goetz). And 20 ft. of it might send the amp into oscillation (=dead). Be careful.
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If you need 20ft of speaker cable, it will be hard to find high quality stuff for less than $350 (sad, but true).
However, I have two possible suggestions. The cheapest would be to make your own flat ribbon speaker cable. I believe the guy at the tntaudio website has a DIY design for such cables.
The alternative would be to find some used Straightwire Music Ribbon. This is the speaker cable I use and it is much sturdier than the Nordost and less expensive. (Maybe I'm just paranoid, but to me, the Nordost is too fragile to be placed under carpets or rugs and constantly be walked on.)
Sonically, I really couldn't detect much of a dropoff from the Blue Heaven, but I'm using very short runs. I do know a dealer who may still have enough stock for a 20ft run. He was the one who originally alerted me to the surprising quality of this cable and, as of 2 years ago, he still had a fair amount. (He said he bought as much as he could when they discountinued making it.) I suspect the cost would be around your budget too. Just email me if you want the dealer's info.