Generally, the answer is 50' interconnects, and balanced interconnects if possible.
However, your results indicate that perhaps only experimentation with both speaker cables and interconnects will provide a satisfactory answer.
Low capacitance interconnects can be purchased or fashioned from Mogami 2534 or Gotham Audio GAC/4-1 microphone cable. I also use Oyaide PA-02 for 30' XLR interconnects with excellent results.
However, your results indicate that perhaps only experimentation with both speaker cables and interconnects will provide a satisfactory answer.
Low capacitance interconnects can be purchased or fashioned from Mogami 2534 or Gotham Audio GAC/4-1 microphone cable. I also use Oyaide PA-02 for 30' XLR interconnects with excellent results.