unsound, by "feedback from the speakers" would you be referring to back emf?
Do you have any technical background, by the way? I'm wondering if you actually know what a zobel does.
Let me restate that the reason amp designers put networks at the output of their amps is to prevent the amp going into oscillation with reactive loads. Many designers feel that the sound of the amp is more open without the network but, of course, this makes them more suscepible to oscilation from very reactive speakers or, perhaps, low inductance cables.
I fear I am belaboring the point but I keep getting the impression that I'm not getting that point across. Perhaps you are thinking of a zobel as a "tweak" rather than a bandaid. When speaker designers build the passive crossover networks for their speakers they can incorporate elements into the design that would effectively be a built in zobel. I would have to assume that if the speaker designer did not use this technique it would be because it was felt that it changed the voicing of the speaker in undesirable ways.
So if your amp doesn't have it and your speakers don't have it, it's likely because the designers didn't want it. So, wouldn't it be better to contact Threshold, in your case, tell them what speakers you are trying to drive and ask them to recommend a cable that would work most effectively in that configuration? And/or, you could talk to the speaker manufacturer. System synergy and component compatibility might be improved, i.e. it could sound better.
Then again, maybe you're satisfied with what you currently have. I am just trying to clarify my reasoning.